If the water stops when you turn the isolator switch off I would say the power is bypassing the shower switch (possible dirt in the contacts stopping the circuit from closing or just burnt out), if water still comes out with no power to shower the solenoid may be getting stuck (usually slows down over time and then stops after about five minutes}
If in doubt either get a pro in or fit a new shower (a new shower with a larger footprint will hide the problem with the tiles) water and leccy dont mix
