It was a low speed corner which I scraped to a halt on! Still somewhat unsettling to be driving along and (effectively) have a wheel come off. Had hit a pothole a while before which I suspect was at least partially to blame (from what I can see at the moment it looks like a clean new break rather than an older crack but I'll be able to tell better when I get it all off.
Fortunately I was not blocking the road - I was quoted some ridiculous wait time from the AA, but in the end they turned up pretty quickly and strapped it back together enough to get home.
I've just ordered the first set of parts from James Paddock, will get the bits off the passenger side and see what else needs doing, then I'll do the trunnion on the drivers side and see what that's like and what needs replacing there.
I saw people recommending a Wanner grease gun so I've just bought one of those from ebay.