That sounds exactly like the job I've just been asked about, only it's shower room walls rather than floor. It's even in a similar area (SW London) so maybe it was the same
Tiler! Also this is apparently only a 2yr old house and the builder/
tiler is taking no responsibility for the workmanship.
If I take it this will be my first job too. I've not seen it yet, only talked over the phone, but I've been told that the wall is damp in the next room too.
I've already advised the customer that the tiles will need to be removed and the walls dried out and treated before tanking and fixing, but he wasn't too happy about the cost implications. However, I think I've persuaded him that the work does need doing if he doesn't want the same problem to arise.
I'll be talking to him again at the weekend so would appreciate any tips that you might give.