Set Square wrote:
Has the pump ever been removed/replaced? Could it have been put back the
opposite way round from how it was when it worked reasonably well?

As long as we've lived in the house, the pump has never been removed or
replaced. I will do what John suggested and get under the floor to check
the flow direction and report back. I'll also flush all the upstairs
rads, and let you know about the flow rate from each valve.

Thanks again and regards,
Bob