Thanks for the help, no leaks apparent around the cylinder, though it is lagged. I have mostly noticed the overflow in the morning first thing, then it stops when we are all up and about, obviously drawing off water. the header has been so hot at one point the water was steaming, and it being plastic was distorting. That'll be a plumber getting a call soon then. Some of the rads are'nt particularly hot after a few days even though I've bled them. Whatever was wrong with a coal fire and lots of jumpers! :bang: