It would most likely be the diverter valve diaphragm not operating the microswitches that bring the hot water on.
You can use a diaphragm from Alpha boilers as a replacement.
Good luck in finding other spares however, the boiler is a Travis Perkins import sold specifically through them, I have never tracked down the actual manufacturer.
A few parts are akin to a Worcester CDI but getting parts is very ad hoc
Carefree, February 2011
