Hi Paul,
Check the non return valve is in place and sealing correctly if ok remove the motor and check the membrame is undamaged and not distorted it should not be brittle if it is replace it also by lightly pushing up the membrane the microswitch should be heard to click easily if not it neeeds replacing,
either call an engineer or buy a cherry microswitch from maplin and have a go beware the two white membrain retaining clips are easy to dammage and they only refit one way around due to the shape of the clip it's not aparrent at first untill you look closley (angled side points down) if you break them clean up the area where the membrane fits and apply silicone sealant sparingly and then fit the membrane leaving over night to seal and cure..
IMPORTANT BIT if the sealing is not done right and water leaks in between the membrane and the opperating foot the electrical chamber will fill with water & result in the machine blowing fuses or shorting the electrical supply.
If your not prepared to take the risk call a SFA engineer a telephone number can be found on the silver rating lable on the back or side of the machine.
Muncher
November 2014