Megaseal removes the grout and replaces it with their product to seal the tiles.
If the pipework inside the wall is leaking they will not stop your problem.
You need to have the shower combination, waste, and tapware checked before spending lots of money sealing the tiles. If the combination is the problem then it needs replacing, which will mean removing tiles, wasting the Megaseal.
Get a plumber in if you feel you cannot test these parts youself.
Email me if you want more information