A possibility which no-one else seems to have mentioned is that the
pump may be controlled by the boiler

If the boiler does do this, and if the thermostat within the boiler
which provides this function fails shut, the pump *will* run
continuously

Could be. I think it's an overridden/failed valve at the moment, but I'll
see if I can figure out if the boiler is in control.

Cheers,

Al.