We have a leak that has appeared on and off for many many years.. It was a problem when we moved in 15 years ago and has reappeared 3 or 4 times since including just recently after an extensive refurbishment of the bathroom and exposure of the back wall. Various plumbers and trades people have looked and exposed the bath, shower, piping etc and found nothing. This is causing major problems and damage now. It seems to be worse after / during shower but interestingly only sometimes. Any advice so very very gratefully received... Signed a desperate (and not in that way) woman...

Hi Desperate and welcome to The Plumbing Page of AMHD.
First a little more information.
What type of shower do you have?
Custom tile or plastic floor and enclosure? If all your pipes have been checked I think I already have a solution for you but it will have to wait until learn more. Back tom you, Tom