On Saturday, I dried it completely with a towel. Then put the lagging over it. I didn't tape the lagging in place, so the seam is split. Last night it was wet to the touch again.
I think it's a leak.
update: I just called Lewisham. As this is a communal pipe, they have to fix it. GET IN.
Strip all the lagging off, dry the pipe with towels, identify beyond doubt if it is leaking or if it is condensation.