After waiting ages to pull out from a busy junction, I forgot to put it in 1st and gave it a short thrash, about 40k revs, in neutral. After that it had intermitent power loss/juddering all the way to work.
On the way back, the problem was the same but with added thick black smoke bellowing out - embarrassing in rush hour. This stopped about half way home.
Any ideas what's happened? Cheers.