After I posted the above update, things got flaky again. Backups were failing and sync was back to being unreliable. One thing I don't think I've mentioned is that whenever sync was 'failing', it was always calendar and contacts that failed (i.e. sync with Outlook 2007); music and apps would continue to sync just fine.
Calendar & contacts sync are my two 'must haves'. I can manage apps manually via the iOS App Store and I rarely listen to music on my iPhone. I was on the verge of taking Jim Turley's advice and switching over to an Exchange ActiveSync-based service, when something happened...
I uninstalled Norton AntiVirus.
Others have highlighted Norton as a potential issue earlier on in this thread and I discounted the possibility because suggestions were focused on it being a firewall issue in the fuller Norton Internet Security suite, whilst I only had the a/v component; I use the Windows firewall, and disabling that made no difference.
However, when I switched from Norton to a competitive product my iPhone immediately went back to syncing everything, all the time. This has lasted two weeks so far, which is a good sign. Might be relevant to anyone still having problems. Removing/switching a/v isn't a trivial task, but it could be worth testing.
Whether this is a permanent fix or whether I'll be back here again in a couple of weeks obviously remains to be seen...