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Thank you ELVIS68, bookbinder, Gizmonster and carrust.
I'm convinced the problem is with the sealant around the bath because surely, if it was the wastepipe it would also leak when the bath itself is being used? This, however, is not the case, it's only when the shower is being used. Plus, in the kitchen, the part of the ceiling where the water is dripping from, and the wall that it's running down, corresponds with where the sealant is split around the bath. Also, my occasional attempts at resealing the bath have stopped the leak for a little while, which would lend support to the sealant being the problem, although I would also be inclined to think a contributing factor to the problem recurring being my DIY skills leave a lot to be desired. Don't get me wrong, I'm not completely ruling out the possibility that part of the problem could be the way the tiles have been fitted.
Yes, I will be looking to buy a good quality silicone this time, such as the Dow Corning 785 Acetoxy Bacteriostatic Sealant as recommended by bookbinder.
Thanks again. :o)