Got it reassembled, and put in some nice ceramic bearing jockey wheels while I was at it.
Seems to be ok, but not ridden it yet.
It was hard to see if there was any bending in the old plates, but there was quite a lot of metal worn away from when the chain had kept coming off the jockey wheel a while ago, which most likely affected shifting. I needed to change the indexing a fair bit with the new plates, which I took to be a good sign.
I'll only know if it's properly fixed when I try and get up a 1 in 5 standing on the pedals in bottom gear.