Hi Andy,
I'm guessing you don't have that little silver card with the radio code on it. Anytime you disconnect the battery you will need to re-enter that code to get your radio working again. It's an anti-theft thing.
Do a forum search for "radio code" You may find a helpful link or two. Failing that, I think the dealership can get you the code, but (of course) there will be a fee for the service.
Try this link for a manual:
http://www.cannell.co.uk/Manuals.htm