I have just moved into a new house, and the first time the bath was used, a small damp patch developed on my kitchen ceiling directly under the bathtub. I have had the plumber out to repair it 3 times, the first 2 times he said he could find nowhere a leak at all, and the 3rd time he came out, he told me it looked like it was possibly an overflow, waistline problem, which he insisted he had fixed. Following the plumbers advice, I immediately went to B&Q and bought some "Thompsons Stain Blocking Damp Seal" and applied to the stained ceiling which by this time had turned yellow with a small black patch in the middle. The Damp Seal sprays onto the stain and turns it white, ready to emulsion over. Now we still use the bath as we have no shower and cant afford one just yet, so I painted over the stain once dry, and all looked perfect, after one week I noticed that the small black patch is starting to come through only it looks a faint grey color at this point, I felt the ceiling and it feels dry (although not as dry as the rest of the kitchen ceiling) and the black patch which I presume is mould is bone dry, but still coming through a bit, any ideas how I can make it go away? it looks ok now but worried it may get worse.