Sorry for starting a new thread, just wanted to say thanks to everyone that helped.

I found a valve that was open, that is tee'd off before the motorised valve. It looks like it diverts some pressure away from circulating around either the heating or hot water and puts it back down towards the boiler.

I've closed it a few turns and now the last radiator is getting warm and the rest are boiling! It's just a question of balancing now.

Thanks again,
Jon

That'll be the bypass valve - it's there to provide a certain minimum flow whenever the pump is running, but there is not full demand (thermostatic valves closed, etc.)

It sounds like you've cured your problem, but a better solution would be to replace it with a differential valve that would open when necessary, but not bypass anything when not required. They're only twenty-odd quid IIRC.