I noticed a slight damp patch near hot water cylinder ... it would only be there on rare occasions ... finally found the cause.
There is a vertical 28mm zone valve (220V actuator) on primary loop .... and a slight leak from there.
It is not leaking from either of the compression joints ... so must be coming out via the central ball valve actuator.
It's a Cimberio 28mm valve one of these:
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On page 161 (3rd page in) it shows an Oring & gasket kit .......... anybody ever serviced one of these valves in-situ
I'm assuming parts will be available.