Sime Super 90 - heating comes on when hot water truned on?

My Sime super 90 has these problems:

* always heats the central heating when the hot tap is turned on

* it takes ages for the hot water to come through the tap

* I can't turn the tap on very much or water begins to cool, and I certainly can't have 2 taps on

* turn tap off and back on again quickly, and water takes ages to heat up again

* when the central heating is on (on purpose) the hot water arrives at the tap a lot more quickly (although still slow)

I did a test of the diaphragm in the diverter valve, and it rises and clicks the microswitch, but could it still be the valve isn't shunting the water where it should?

Any help on what's going on greatly appreciated.