Boiler cuts out when CH set to on?

I have a potterton system with a 2002 timing controler and a PT2 remote electronic thermostat. If the system is set for hot water only the boiler fires in correct sequence and works perfect, however, when the ch is set to on the boiler fires in sequence then the gas cuts off and the burner stops. If I turn back to HW only the boiler and pump etc works fine. Any ideas? The potterton manual syas it could be the elctronic control unit.