Thanks Howard, I haven't decided what to do about it yet and suggestions are welcome.
Interesting experiment today. I used a crowbar under the stand to tilt the lathe to see if the level showed any sign of the bed untwisting. Not at the time, but retesting after removing the crowbar showed the twist had nearly halved. Cutting another test bar showed the taper has also halved (roughly). I think lifting the lathe moved it slightly on the rough floor and the bed's less twisted in the new position.
Health and Safety note: lathes are top heavy beasts. I think it would be easy to tip mine over with a crowbar! Maybe the improvements should include bolting it down.
Dave