I got the first section of coving taken down. I had to cut the top of it because the ceiling tiles covered the top, but the cut will be hidden once the corner trim is put back on. So now I've got a nice big gap in the coving at the top of the cupboards, I pulled down some more lath and plaster wall, found a few more gaps in the wall that are letting in nasty drafts, but ran out of time to finish the job before my wife needed the kitchen back so I stuffed a few wads of fiberglass in strategic places and cleaned it up. The coving survived being taken down so I should be able to put it back up more or less exactly as it was. It was surprising just how many nails pinned it into place. When it goes back up the chances are I'll just put a small number of white-headed trim screws in it to hold it where I need it.
Tomorrow I have a big bag of salvaged cellulose I can feed into the wall cavities, and hopefully get some of the gaps in the wall filled. Some of them are behind studs, which makes it more difficult. If needs be I can use spray foam to fill them, although I'd rather do a nicer job than that.
It already feels warmer in the kitchen. A couple of power sockets that have had cold drafts coming out of them for ages no longer have cold drafts coming out of them. Hopefully this will save us another chunk of cash through the winter, there were parts of the kitchen that got unpleasantly cold.
Since this looks like it has made a difference I'll most likely take off the next piece of coving. I wasn't sure whether I'd really want to be doing that, but I think it will be worthwhile.