Quite honestly, I don't know. One minute I was happily enjoying the countryside, and the next, the rear wheel stopped going round (and the pedals, which was the bit which surprised me more at the time).
I was about halfway into my 7 mile commute, but luckily could phone the wife and ask her to pick me up in the car... I gave up pushing / carrying the bike after half a mile or so because the rear wheel was locked solid, and the tools I had on me weren’t man enough for the job.
I took the bike to the LBS just now, and have it booked in for next week, but it looks like (maybe) the chain broke and took the derailleur with it, or maybe the derailleur simply went too far, and hit the spokes. I reckon it was probably the chain because I heard a clicking noise (in time with a chain rotation) just before it happened. Yes, yes, I should have stopped when I heard it!
I had something like this happen to me in the middle of nowhere (Usually does with this sort of thing!! ) The result: New chain, rear dérailleur and new rear sprocket. Being a steel bike I was able to re-bend the drop-outs to make them straight again. I was damn lucky my rear wheel escaped being damaged, that would of probably pissed me right off as I had lovingly built it that weekend.