I have this fixed now. The 404 error was due to some kind of error on my host - the control panel was reporting
'4.0 (Integrated)', but in fact it was using '4.0 (Classic)' - something had got confused when my host moved a static ip address from one of my domains to another.
After that was fixed I got a 500 error. This has been resolved also - the app did not have write permissions to the App_Data directory.
All working now.
Thanks bertrandleroy, your comments were right on the mark.