DrByte,
I included your code in application_top in both admin/includes and includes itself. Both the site and admin now work.

I now doubt the truthfullness of what the hosting company told me (that they had changed nothing). They only supply email support at weekends so I will have to wait until Monday to speak to a live person.

There isn't a configure.php in admin/includes and, from my backup, don't think there has ever been one.

Many thanks for your help!