low pressure light comes on and the water stops

It may be your solenoid valve, you can check it if you have a multi meter, you should have a reading of at least k3.5 ohms, across the two terminals any less and you need a new valve. (Check the valve in the failed state.) (When it has stopped working!)

If there is power getting to the solenoid and it is not operating then there is a good chance that you have a faulty solenoid.

http://www.youtube.com/watch?v=NzJjzWqxesc

The video is intended as a guide and does not cover every shower.

If you don’t have a multimeter  we sell them  with a guide on how to use them for checking and testing the various parts of your shower.

Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses and letting you know much power you have left in batteries

(So you will know if they are good or not) and lots of other jobs, in short they are well worth a few pounds

http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester

I think you may get away with only fitting the coil

The normal fault with a solenoid valve is the coil breaks down, we can supply the coils separately however you have to be careful when changing them over.

Turn off the water and power then place two screwdrivers between the base of the coil (usually brown or white) and the base of the solenoid body and ease the old coil off the body.

On some units the coil can be replaced without removing the existing solenoid from the shower case on others it will be necessary to remove the valve to fit the new coil.

http://www.showerdoc.com/solenoid-coil-universal-no-valve-incl