WM I had this problem in my London flat. Turned out, after massive disruption and much plumber time spent, that the waste needed screwing in tighter to the bathtub. A 30sec job. I was washing the downstairs kitchen every time I had a bath or shower. They had to replace their kitchen and ceiling and we had no idea it was still a problem as our landlords plumber had left, until the downstairs builder knocked on our door and said "wtf are you doing up their?" I'd just run a bath after getting home so was easy to track. This was after the plumber had been there for a week "fixing" it. Builder came upstairs and screwed the waste in, voila, problem solved.
I'll never forget standing in their kitchen watching the waterfall.