There are certainly no radiators between hot water return and boiler.

I will take the valve out and check that it is closing properly.
That'll have to wait until I have time to drain the system It is quite
possible there is some crud in the valve as when I changed the pump it
was full of crud. So much so that I was amazed it was working at all.
The aperature size that was left for water to flow through was about 5
mm at best.

Thanks for the advice.

AJJ.