I've had this problem before and I got around it by adding 'NT AUTHORITY / NETWORK SERVICE' as a Login then creating a user on the DB and mapped it to that login. However, I'm now using SQL Server 2012 (Express) and I don't see a way of mapping a user to a login. Anyway, by what I've been reading, it may just be a problem with the user in my connection string which is:
<add name="MyConnectionString" connectionString="Data Source=USER-PC\SQLEXPRESS;Initial Catalog=UKPR;Integrated Security=True"
When I run a query in SQL Server to find out who the owner of the DB I'm trying to access is, it says 'USER-PC/USER' so what is wrong and what else do I need to do to grant SELECT, UPDATE etc permissions on my DB??