So an update on fault finding as follows - I took out service boiler. Boiler underside may be blackened, but doesn’t seem to show leak source. However source from picture would seem to be on adjacent fill manifold, see left (outer) connection, which seems to be the line from the feed pump. Lots of gunk. So I guess slow leaking when the boiler was being filled only?
I spotted minor leaks is on the steam line, both on the inner side, also on outer side, so two of the connections seems to be leaking, as can see green copper oxide staining. I’ll strip these down, clean them, PTFE tape and reassemble.
First time tinkering with machine after 3years, so far so good :)