I've spent a lot of today going through my sites and whilst there are still a few problems, the majority of functionality is working now.
Hetha's post in
this thread was extremely useful.
In addition, I've asked support to enable register_long_arrays in the new php.ini - this seemed to fix the problems I was having with the Oscommerce Imagemagic contribution. The only other problem I'm having now is to do with a few memory settings (for file uploads, etc.). Hopefully, once I've asked Support to fix them, both my sites will be up and running fully again.
Thanks very much to Support's assistance today.