In message , David W.E.
Roberts writes

Can anyone detect anything screamingly obvious that I have neglected to
do?

Pump is definitely working by the way.

Jules

Have you bled the pump?
Should be a big (plastic?) screw you loosen to let water out.
Air in the pump can prevent water circulating.

I hope you're not on about the big chrome screw at the back of the pump.

This is not a bleed screw.
--
geoff