Started intending to adjust cones on rear wheel. Ended up:
Cleaning entire bike (well, scrubbing most of the mud off).
Adjusting cones
Removing jockey wheels, cleaning gunk, regreasing etc,
Ditto derailleur (oil not grease)
Discovering why chain kept dropping - it was far too long and front derailleur was miles out - clean entire chain, can't find removable link, nevermind I have another, remove two links. adjust front derailleur
Adjust rear derailleur - wow it changes properly now!
Cleaning up jockey wheels must be the most grimy job there is on a bike.