This problem is caused by corrosion on the contacts of the switch, on the bulkhead. It doesn't take long for this to develop (my GTS is just 6 months old and had already stopped working). To fix it you need to clean the switch as follows:
1. Remove the black cover from the left side bulkhead.
2. Reach behind the switch and while applying a little pressure to the top and bottom of the switch, push it outwards (tricky but possible).
3. Pull out the wiring plug from the back of the switch.
4. Open the switch by pressing on the tang illustrated in the photo below and doing the same on the other side of the switch.
5. The part of the switch with the button has two nipple shaped contacts which will almost certainly be dirty - clean them with some fine sandpaper or wire wool etc.
6. The other part of the switch has a metal insert on one side (of the inside) - clean these in a similar way.
7. Make sure the spring is still located on the peg in the main body of the switch and push the two parts back together (also make sure the metal pins on the button part are at the same side as the contacts on the inside of the main body.
8. Reconnect the wiring plug and test to make sure the switch is working.
9. Push the switch back into the bulkhead.
10. Repeat in 6 months!
Nice, easy and satisfying job - Good Luck
Switch pushed out from it's housing
Close-up of the switch and how to take it apart
The switch opened - note the dirty contacts