I have solved the problem but it was blooming hard work. Eventually I gave up trying to sort it on the handset so I reflashed my phone, installed the latest one again via extras-devel, then it didn't like the settings it remembered so I deleted the accounts and set them up from scratch.

That worked but I'd love to know why the problem happened in the first place, this deleted all my calendar events and it's only lucky that I had another device that hadn't sync'd to google so I still had them, if it wasn't for that I would have lost everything.