Hi Ray. It actually wasn't a server error, it was completely my fault.
Just for the record if anybody else needs this...
I had copied some old code across to start my page, and I thought I had removed all the mysql php, but I left an old mysql query on the page. the mysql query had no reference to connect with, and therefore gave an error that looked like a server issue.
Moral - check your code and check again!
Thanks for your help!