Hi having same prob with mine. Check you have 4 - 5 volts on pcb X4 pins 10-11. If so, thermistor fine. Is micro switch switching on? Turn on hot tap and put a wire link accross 2 pink wires on pcb. You might need a small mirror to watch the pin on the divertor valve come out and operate the switch. If it tries but not far enough, try removing/cleaning the inlet flow regulator. 24mm brass nut, back right hand side. Contains filter and reg. Shut off cold inlet first and open all hot taps to drain. Reg is the green button with cream cog shape in middle. throw it away and re assemble.
Next, drain whole system and remove 27mm nut holding DHW switch pin on valve. Careful, strong spring and small parts inside. Clean all, lubricate with silicone and reassemble. Also try flushing system out. Think I've cured mine. Time will tell!!
Billy
December 2008