desgna ds 3000 shower fault

If the light in the isolation switch goes out when you try to switch the shower on then the isolation switch is probably faulty

If the light is OK then the next thing to check is the TCO see below

I think you may have a faulty Thermal Cut Out 

A faulty TCO will have no continuity across the two terminals (The black thing on top of the heating tank) 

Therefore a good TCO will have power at both connections and a faulty TCO will have power at only one. 

(The function of the TCO is, if the water over heats it cuts the power off to the elements)

See Video.

http://www.youtube.com/user/showerdoc#p/u/2/_zi8JbEMYMs

If the TCO is OK then check the solenoid see below.

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

 If these checks are all OK then get back to us with the results.