Heating does not come on again after running hot water tap?

BAXI 105e Instant. Can only get heating to come ON after switching OFF at mains switch for approx 10mins & switching back On. Heating is ON and then when I use hot water tap, heating switches OFF as it should but does not come back ON again after I stop using hot water. Tried running pump outside boiler,resistance across sensors measured, resoldered PCB connections,drained & refilled system,microswitch resistance,motor on top of valve.