I have traced it to the plumbing to the upstairs washing basin - the 15mm pipe to the hot water tap appears to be leaking at the last 90-degree Yorkshire elbow before the tap.
Emergency action had been to turn off the valve providing water 'upstream', and put a container under the elbow; lifting up the pipe (which was 'dangling' in mid air a bit) seems to have helped for now.
Tomorrow I will have to look at a proper repair. I have never had a joint fail on me before (the soldering for this was done around ten years ago, not by me ;-/)
It is possible/sensible to try to remake the joint, simply by holding the pipes in the correct position, and applying heat and solder? Or is it necessary to pull the joint apart (any tips on this?) and remake with a fresh elbow/solder?
Thanks for any advice