It maybe your solenoid valve, you can check it if you have a multi meter, you shouldhave a reading of at least k3.5 ohms, across the two terminals any less and youneed a new valve. (Check the valve in the failed state.) (When it has stoppedworking!)
If thereis power getting to the solenoid and it is not operating then there is a goodchance 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 youdon’t have a multimeter we sell them with a guide on how to usethem for checking and testing the various parts of your shower.
Apart from helping to fix your shower, they are really useful forchecking you have a power supply to appliances, for checking fuses and lettingyou know much power you have left in batteries
(So youwill know if they are good or not) and lots of other jobs, in short they arewell worth a few pounds
http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester
Unfortunately with this shower it may be a number of other parts as well and all you can do is replace each in turn or opt for a different shower.