I have a Vaillant VCW-sine T3 gas boiler - the domestic hot water works fine
but not the central heating.

The pump is free to turn with a screwdriver through the drain cover. When
CH is 'demanded', the pump has power applied and if I listen with a
screwdriver as a stethoscope it's whirrs so I assume it's okay. The boiler
also fires up but shuts down again after a few seconds when the sensor heats
up. It can be restarted (carefully) by pulling the wire off the temperature
sensor.

The pump gets hot, the pipework at the top of the boiler gets hot but the
flow pipe remains cold. I've bled at the top of the boiler and at the pump.
May not be relevant but it seems to have developed a slow pressure leak in
recent months.

Views appreciated. Is this a symptom of divertor valve failure? Any
insight into how these work?

Thanks
Phil