Turn off your electricity at the fuse box.
Locate and unscrew the housing screws on the top and bottom of the outer casing.
Pull the front of the casing away and place somewhere safe.
On the inside you should see a copper-ish coloured drum with lots of wires attached to it located towards the left hand side of the box. There should also be a thin flat silver plate attached to the top with small nuts.
Take a small spanner and remove the nuts. Keep these safe.
Now, the thin flat silver plate should lift out revealing a 1cm-ish deep cylinder which is recessed into the cooper coloured barrel.
Place your finger under the cylinder and press gently, you should hear a quiet 'click'.
Now, starting with the nuts, put everything back together making sure you don't remove anything else or forget anything.
Once the casing is back on turn your electricity back on at the fuse box and test the shower - if you've done it correctly the shower should work perfectly.
This problem will reoccur and you will have to repeat this proceedure. Worst thing is, when it's usually time to take a shower (in the morning before work) this is usually when you won't have time to do this.
Best bet, get a new shower.
Stewart W, September 2008