Poss leaking shower behind the wall grouting is black - Plumbing job in Berkhamsted, Hertfordshire

Job Description

I have a fitted shower in my cottage bathroom that I rent out. The agent has reason to believe there is a leak behind the shower wall. I do not want to strip the tiles off, the only way to look at the problem apparetnly is to cut a whole in the wall on the landing behind the shower. The people that rented the cottage let the grouting on the tiles go black, I dont know how to fix this canyou help please