It is taut and pink: no hair, and seems to have blood vessels on the surface. It seems to have come up very rapidly, unless I've only seen the lamb from the wrong side for ages! No particular signs of pain, but it is itchy: he scratches at it, which I suppose could also indicate pain.
I'll contact the vet in the morning. I expect I'll be give long acting antibiotics and will be told to lance it, but he might ask me to take him in (if I can catch him).