The Jupiter 135/4 lens seems to be working a bit better after turning it backwards and forwards a few times, which makes me think the problem was simply lack of use and grease badly distributed in the works. Still very stiff towards infinity. I've added it to my ebay listings, starting at 99p but hoping it will do considerably better. It turns out that these lenses have a weird interchangeable mount and I could actually convert it to Nikon fit for a tenner, but my D50 doesn't play nicely with manual lenses and I already have a zoom covering that range so it's probably more trouble than it would be worth.
update - first bid within an hour or so, hopefully more to come.