Intermittant H/W again

I have been trying to fix my boilermate 2000 for a while now, and have recently stumbled upon your forum, here is the story so far

Initially I had intermittent hot water, which then became no hot water. I put my engineers head on and had a look at the electronics and read the manual and figured out that the PHE sensor had failed. I replaced the sensor and got back to intermittent hot water, looking at the control board, sometimes it realises that hotwater is being 'called for' and it illuminates the lower segment on the 7-segment led, and sometimes it does not. other times it will start supplying hot water (led illuminates and DHW pump starts) and part way through it will switch the LED and the pump off despite the shower or hot tap running.

The conclusion I have come to is that their is (probably) another sensor failure. Does anyone know how the boiler detects that hot water is being 'called for' ??? I was expecting to see some kind of pressure switch, but looking at the inputs to the control board, I cannot see one. The only other possibility I could think of was one of the other temp sensors looking for a drop in temperature, but thus far have failed to track it down.

Any Thoughts?

Paul.