How do I reduce the cold water flow?

I have a Gainsborough 9.5E electric shower which has recently 'turned cold'. We can only get cold water (which is at a very high flow). I've tried the troubleshooting tips in the manual of very slowly turning the temperature dial into the warm section but this only results in a slight increase in temperature. The shower is still heating the water as when it is switched off the cold flow is removed and you get a few seconds of very warm water before it finally stops. It seems that if we could reduce the cold water flow there may be a chance of some warm water?