I looked at the permissions and the ones on my account are fine. I checked the full control permissions on all accounts (just in case some illogical step might work!). Still not working.

The first time I restarted after fiddling around with other users permissions, my doc still opened as read only. I restarted again for other reasons and when I opened my doc, hey presto, I'm back to [compatability mode]. Have no idea why or if it will last...