I have just changed the pump on our system and now when the heating comes on it works fine for a couple of hours or so, a typical timed heating session. However if it is swithched to "constant" the system switches itself off. The Rads go cold. The pump is still working and there is loads of hot water. The boiler is an old Glow-worm Fuel Saver. It is what I would term a traditional system with a hot water tank and a header in the loft. The room thermostat and boiler thermostat are on max. I have been making sure the rads are bled. Could it be the diverter valve? Help please with suggestions.