A friend is having trouble getting his downstairs radiators hot. (Upstairs
it balanced down to a minimum)
A plmber has replaced the pump to no avail. Today he did a power flush but
he connected to a radiator flow and return. I had assumed that flushing was
normally done at the pump connectors.

If there is a blockage then I believe the pipes under the floor are the
problem.

Sould he be trying to connect the power flush into one radiator - and the
return into a different radiator?