Cracked it! Took your advice and tried a simple report it worked.
I then looked closely at the reports that did not work. Guess what? They had been set to Basic Syntax by mistake. Put them back to Crystal syntax and they worked.
What I don't understand is why they ever worked in VS 2003.
As they worked in VS 2003 and not work in VS 2005 I assumed it was a VS 2003 -> VS 2005 issue.
Dave