PreImage doeant appear to be working

CRM2013

I have an update plugin on a custom entity. I have the need to reference the data before the record is update. I was pointed to using images to grab the data before the record is updated. Using examples from the net, I tried creating a plugin. This doesn't appear to be working, so I have cut it down, to get a working solution. Following the examples, I can not see where I am going wrong . Can anyone shed some light

Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports Microsoft.Xrm.Sdk
Imports System.ServiceModel
Imports Microsoft.SqlServer
Imports System.IO
Imports CRM2013_OriginatingAccounts.clsGlobals
Imports Microsoft.Xrm.Sdk.Query
Imports Microsoft.Xrm.Sdk.Messages
Imports CRM2013_OriginatingAccounts.cls_SQLFunctions

Namespace OAccountsUpdate
    Public Class cls_OAccounts_update
        Implements IPlugin

        Public Sub Execute(serviceProvider As System.IServiceProvider) Implements Microsoft.Xrm.Sdk.IPlugin.Execute
            Dim context As IPluginExecutionContext = DirectCast(serviceProvider.GetService(GetType(IPluginExecutionContext)), IPluginExecutionContext)

            ' Variables
            Dim strLicence As String = String.Empty
            Dim strOAccountID As Guid = Nothing
            Dim intOAccountClientID As Integer = 0
            Dim strCountry As String = String.Empty
            Dim strBACSID As String = String.Empty
            Dim strSortCode As String = String.Empty
            Dim strAccountNumber As String = String.Empty
            Dim StrAccountName As String = String.Empty
            Dim decDailyLimit As Decimal = 0D
            Dim decWeeklyLimit As Decimal = 0D
            Dim intVarPeriodDays As Integer = 0
            Dim decVarPeriodLimit As Decimal = 0D
            Dim decMonthlyLimit As Decimal = 0D
            Dim bolAllowPayments As Boolean = False
            Dim bolAllowDebits As Boolean = False
            Dim bolActive As Boolean = False

            Try
                If context.PreEntityImages.Contains("preAccountUpdate") AndAlso TypeOf context.PreEntityImages("preAccountUpdate") Is Entity Then
                    Dim PreUpdateEntity As Entity = DirectCast(context.PreEntityImages("preAccountUpdate"), Entity)
                    strOAccountID = New System.Guid(PreUpdateEntity.Attributes("new_accountkey").ToString)
                    intOAccountClientID = CInt(PreUpdateEntity.Attributes("new_accountid").ToString)
                    Throw New InvalidPluginExecutionException(strOAccountID.ToString & " ," & intOAccountClientID)
                End If


            Catch ex As Exception
                Throw New InvalidPluginExecutionException(ex.Message)
            End Try
        End Sub

    End Class
End Namespace

Dont ask me .. i dont know