I dont agree ... A Valiant pump will be perfectly adequate. It may be a Grundfoss.
I reckon he has a big airlock in the pipework somewhere. Maybe in the pipework under floor upstairs or one that connects up the cold rad.

Try shutting all the rads that heat up OK, charge system pressure up to 2 bar ... Open the rad bleed valve .. after ensuring the lockshield and control are both open.
Try to give a thorough bleed. I like to see hot water coming thru at this point. Keep the pressure topped up while you do this.
Try this on the other rad that doesnt get hot, see what you can achieve mate.

There are other more drastic measures to get air out including back-filling .. dont wanna do that yet tho.