I have a radiator with a sluggish flow pipe (checked by looking at flow
from radiator drain when opening either flow or return and seeing a vast
difference)

Based on a previous problem I suspectthe outlet from the manifold to be
choked with scale and or debris.

The manifold is rather "busy" and in a tricky location. I don't want to
just go in randomly disturbing joints.

Any ideas for identifying which union it might be. I can possibly eliminate
some from the direction - but want to identigy the problem one.

I had previously had a radiator re-piped (during some building work) due to
the same problem - but the blockage was only at the manifold - the pipe was
clean. Unfortunately I wasn't home when it was being done.