(When you turn the thermostat up you can hear the pump start, but the
rads are hot anyway)

Possible solutions:

1. Stick a zone valve on the heating circuit.

2. Stick a gravity (non-sprung) non return valve in the system. This may
require you to reverse the pipes (or simply pump direction if external to
boiler) if the pump is currently set to assist gravity convection, rather
than oppose it, and the convection flow is strong, as the convection flow
might actually be strong enough to open the NRV.

Christian.