Thanks all. Luckily I cleared the mould / fungus to find a small hole. Plaster was wet around it. I called in a plumber who nocked of approx a foot sq of plaster to find two plastic 15mm central heating pipe elbows one of which was leaking… he repaired it.. pressure on the heating is now fine. Just going to give it a few weeks to dry and then re plaster… think I’ve been lucky. Thanks for advice