While the upgrade itself went pretty well there seem to be some really odd issues affecting this site. All the other sites running of this MT install are behaving fine, but this one is having issues with templates.
I'm currently trying to work out what the best course of action is, so please excuse the really ugly search results pages if you conduct a search.
I'm considering nuking the templates entirely and reverting back to a default template set before re-adding any of my own minor customisations (and they really are quite minor)
I suspect that I have some old template code floating around that is causing conflicts, but whether I will be able to isolate or not is doubtful - especially as it hasn't happened with any of my other blogs, which is truly bizarre!
Thanks to Donal for highlighting the issue for me.
Update: Nuking the template seems to have solved the problem. Of course I've now got to go back and customise them ... or maybe I can just wait as someone promised me a new design!
Update 2: Due to my own stupidity I managed to disable the single entry archives, so they weren't being created! That's now fixed.