diff --git a/ClearBank.DeveloperTest.Tests/ClearBank.DeveloperTest.Tests.csproj b/ClearBank.DeveloperTest.Tests/ClearBank.DeveloperTest.Tests.csproj
index b21ab73..bc0ae4b 100644
--- a/ClearBank.DeveloperTest.Tests/ClearBank.DeveloperTest.Tests.csproj
+++ b/ClearBank.DeveloperTest.Tests/ClearBank.DeveloperTest.Tests.csproj
@@ -3,10 +3,16 @@
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
     <IsTestProject>true</IsTestProject>
+    <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
   </PropertyGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\ClearBank.DeveloperTest\ClearBank.DeveloperTest.csproj" />
+    <PackageReference Include="System.Configuration.ConfigurationManager" Version="8.0.0" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.8.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.0" />
   </ItemGroup>
 
 </Project>
\ No newline at end of file
diff --git a/ClearBank.DeveloperTest.Tests/Tests/PaymentServiceTests.cs b/ClearBank.DeveloperTest.Tests/Tests/PaymentServiceTests.cs
new file mode 100644
index 0000000..cbd9524
--- /dev/null
+++ b/ClearBank.DeveloperTest.Tests/Tests/PaymentServiceTests.cs
@@ -0,0 +1,73 @@
+using ClearBank.DeveloperTest.Data;
+using ClearBank.DeveloperTest.Services;
+using ClearBank.DeveloperTest.Types;
+using System;
+using Xunit;
+
+namespace ClearBank.DeveloperTest.Tests
+{
+    public class PaymentServiceTests
+    {
+        private class FakeStore : IAccountDataStore
+        {
+            public Account Account { get; set; }
+
+            public Account GetAccount(string accountNumber) => Account;
+
+            public void UpdateAccount(Account account) => Account = account;
+        }
+
+        private class FakeFactory : IAccountDataStoreFactory
+        {
+            public FakeStore Store { get; } = new FakeStore();
+
+            public IAccountDataStore Create(string type) => Store;
+        }
+
+        [Fact]
+        public void Bacs_Fails_When_Account_Null()
+        {
+            var factory = new FakeFactory();
+            factory.Store.Account = null;
+
+            var svc = new PaymentService(factory);
+
+            var req = new MakePaymentRequest { DebtorAccountNumber = "123", Amount = 10m, PaymentScheme = PaymentScheme.Bacs };
+
+            var result = svc.MakePayment(req);
+
+            Assert.False(result.Success);
+        }
+
+        [Fact]
+        public void FasterPayments_Fails_When_Insufficient_Balance()
+        {
+            var factory = new FakeFactory();
+            factory.Store.Account = new Account { AccountNumber = "123", Balance = 5m, AllowedPaymentSchemes = AllowedPaymentSchemes.FasterPayments };
+
+            var svc = new PaymentService(factory);
+
+            var req = new MakePaymentRequest { DebtorAccountNumber = "123", Amount = 10m, PaymentScheme = PaymentScheme.FasterPayments };
+
+            var result = svc.MakePayment(req);
+
+            Assert.False(result.Success);
+        }
+
+        [Fact]
+        public void Chaps_Succeeds_When_Live_And_Allowed()
+        {
+            var factory = new FakeFactory();
+            factory.Store.Account = new Account { AccountNumber = "123", Balance = 100m, AllowedPaymentSchemes = AllowedPaymentSchemes.Chaps, Status = AccountStatus.Live };
+
+            var svc = new PaymentService(factory);
+
+            var req = new MakePaymentRequest { DebtorAccountNumber = "123", Amount = 10m, PaymentScheme = PaymentScheme.Chaps };
+
+            var result = svc.MakePayment(req);
+
+            Assert.True(result.Success);
+            Assert.Equal(90m, factory.Store.Account.Balance);
+        }
+    }
+}
diff --git a/ClearBank.DeveloperTest/Data/AccountDataStore.cs b/ClearBank.DeveloperTest/Data/AccountDataStore.cs
index 89e49dd..a2b8360 100644
--- a/ClearBank.DeveloperTest/Data/AccountDataStore.cs
+++ b/ClearBank.DeveloperTest/Data/AccountDataStore.cs
@@ -2,7 +2,7 @@
 
 namespace ClearBank.DeveloperTest.Data
 {
-    public class AccountDataStore
+    public class AccountDataStore : IAccountDataStore
     {
         public Account GetAccount(string accountNumber)
         {
diff --git a/ClearBank.DeveloperTest/Data/AccountDataStoreFactory.cs b/ClearBank.DeveloperTest/Data/AccountDataStoreFactory.cs
new file mode 100644
index 0000000..4c95b7c
--- /dev/null
+++ b/ClearBank.DeveloperTest/Data/AccountDataStoreFactory.cs
@@ -0,0 +1,17 @@
+using System.Configuration;
+
+namespace ClearBank.DeveloperTest.Data
+{
+    public class AccountDataStoreFactory : IAccountDataStoreFactory
+    {
+        public IAccountDataStore Create(string type)
+        {
+            if (type == "Backup")
+            {
+                return new BackupAccountDataStore();
+            }
+
+            return new AccountDataStore();
+        }
+    }
+}
diff --git a/ClearBank.DeveloperTest/Data/BackupAccountDataStore.cs b/ClearBank.DeveloperTest/Data/BackupAccountDataStore.cs
index 170362c..d00fbaa 100644
--- a/ClearBank.DeveloperTest/Data/BackupAccountDataStore.cs
+++ b/ClearBank.DeveloperTest/Data/BackupAccountDataStore.cs
@@ -2,7 +2,7 @@
 
 namespace ClearBank.DeveloperTest.Data
 {
-    public class BackupAccountDataStore
+    public class BackupAccountDataStore : IAccountDataStore
     {
         public Account GetAccount(string accountNumber)
         {
diff --git a/ClearBank.DeveloperTest/Data/IAccountDataStore.cs b/ClearBank.DeveloperTest/Data/IAccountDataStore.cs
new file mode 100644
index 0000000..258ad77
--- /dev/null
+++ b/ClearBank.DeveloperTest/Data/IAccountDataStore.cs
@@ -0,0 +1,10 @@
+using ClearBank.DeveloperTest.Types;
+
+namespace ClearBank.DeveloperTest.Data
+{
+    public interface IAccountDataStore
+    {
+        Account GetAccount(string accountNumber);
+        void UpdateAccount(Account account);
+    }
+}
diff --git a/ClearBank.DeveloperTest/Data/IAccountDataStoreFactory.cs b/ClearBank.DeveloperTest/Data/IAccountDataStoreFactory.cs
new file mode 100644
index 0000000..428497b
--- /dev/null
+++ b/ClearBank.DeveloperTest/Data/IAccountDataStoreFactory.cs
@@ -0,0 +1,7 @@
+namespace ClearBank.DeveloperTest.Data
+{
+    public interface IAccountDataStoreFactory
+    {
+        IAccountDataStore Create(string type);
+    }
+}
diff --git a/ClearBank.DeveloperTest/Services/PaymentService.cs b/ClearBank.DeveloperTest/Services/PaymentService.cs
index 573713d..9aba5b0 100644
--- a/ClearBank.DeveloperTest/Services/PaymentService.cs
+++ b/ClearBank.DeveloperTest/Services/PaymentService.cs
@@ -6,65 +6,44 @@ namespace ClearBank.DeveloperTest.Services
 {
     public class PaymentService : IPaymentService
     {
+        private readonly IAccountDataStoreFactory _factory;
+
+        // Default constructor preserves original behavior
+        public PaymentService() : this(new AccountDataStoreFactory()) { }
+
+        // Allow injection for testability
+        public PaymentService(IAccountDataStoreFactory factory)
+        {
+            _factory = factory;
+        }
+
         public MakePaymentResult MakePayment(MakePaymentRequest request)
         {
             var dataStoreType = ConfigurationManager.AppSettings["DataStoreType"];
 
-            Account account = null;
+            var accountStore = _factory.Create(dataStoreType);
+            var account = accountStore.GetAccount(request.DebtorAccountNumber);
 
-            if (dataStoreType == "Backup")
-            {
-                var accountDataStore = new BackupAccountDataStore();
-                account = accountDataStore.GetAccount(request.DebtorAccountNumber);
-            }
-            else
-            {
-                var accountDataStore = new AccountDataStore();
-                account = accountDataStore.GetAccount(request.DebtorAccountNumber);
-            }
-
-            var result = new MakePaymentResult();
+            var result = new MakePaymentResult { Success = true };
 
-            result.Success = true;
-            
             switch (request.PaymentScheme)
             {
                 case PaymentScheme.Bacs:
-                    if (account == null)
-                    {
-                        result.Success = false;
-                    }
-                    else if (!account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.Bacs))
+                    if (account == null || !account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.Bacs))
                     {
                         result.Success = false;
                     }
                     break;
 
                 case PaymentScheme.FasterPayments:
-                    if (account == null)
-                    {
-                        result.Success = false;
-                    }
-                    else if (!account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.FasterPayments))
-                    {
-                        result.Success = false;
-                    }
-                    else if (account.Balance < request.Amount)
+                    if (account == null || !account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.FasterPayments) || account.Balance < request.Amount)
                     {
                         result.Success = false;
                     }
                     break;
 
                 case PaymentScheme.Chaps:
-                    if (account == null)
-                    {
-                        result.Success = false;
-                    }
-                    else if (!account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.Chaps))
-                    {
-                        result.Success = false;
-                    }
-                    else if (account.Status != AccountStatus.Live)
+                    if (account == null || !account.AllowedPaymentSchemes.HasFlag(AllowedPaymentSchemes.Chaps) || account.Status != AccountStatus.Live)
                     {
                         result.Success = false;
                     }
@@ -74,17 +53,7 @@ public MakePaymentResult MakePayment(MakePaymentRequest request)
             if (result.Success)
             {
                 account.Balance -= request.Amount;
-
-                if (dataStoreType == "Backup")
-                {
-                    var accountDataStore = new BackupAccountDataStore();
-                    accountDataStore.UpdateAccount(account);
-                }
-                else
-                {
-                    var accountDataStore = new AccountDataStore();
-                    accountDataStore.UpdateAccount(account);
-                }
+                accountStore.UpdateAccount(account);
             }
 
             return result;