On Mon, 02 Dec 2019 09:53:42 GMT, John wrote:

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.

Let the system go cold. Shut control (not lock sheild) valves on all
but the poor performing radiator. Turn system on, monitor pipe temps.
The one that warms first is the flow to the dodgy rad one a bit
cooler the return. Helps if you have an IR thermometer. Re-open the
control valves.

--
Cheers
Dave.