In an earlier contribution to this discussion,
Chris wrote:

One more question, if I were to close off the return valve for all
suspect radiators would this help find/eliminate the leak from these
return pipes? If so this could at least send me to the correct room.

It wouldn't really help - because there's static pressure in the pipes, with
the valves open or closed. In other words there's an alternative route to
each bit of pipe, other than the one which goes through the radiator.
--
Cheers,
Set Square
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