OP here - I have
emotions to report!
Is this common on an internet weatherstation board?
1.
Relief, happiness and contentment. And a stable bank balance.
My trusty VP1 is fixed and up and running again, all sensors working cap'n.
2.
Embarrassment and some shame at my poor diagnosis.
It was an corroded RJ11 coupler that was the problem, nothing to do with the ISS board.
There's long run from the console to the ISS, so there's two cables, joined with a (15 year old) female-female RJ11 coupler in a 'weatherproof' junction box with grommets.
I'd already brought a new 40 metre cable and tried replacing cables 1 and 2 and checked (successfully) for continuity on my network cable tester. What I hadn't done is take the console outside to the ISS and hooked it up with a 2m cable directly - and that's what worked yesterday. Not sure why the cable-tester was reporting things ok - I've got a memory of one of the 4 wires being slightly dimmer on the tester last month.
So one new RJ11 connector later and all is fine. I've now filled every RJ11 port with dielectric grease (coupler and ISS). The outside world can be hard place for IT kit, my cables regularly get chewed by mice and occasionally cows!
I can only say
thank you and
thank you again for all the help and knowledge released in the posts above. I'm sure those pinouts and specs for the power regulator will be useful info for someone in the future.
One positive here is that the 2 month gap in readings has given me an incentive to ditch my Weatherlink setup on a networked Win10 laptop and replace it with a raspberry pi running CumulusMX. Seems much more stable for Wunderground uploads and no need to pay for any Davis subs, quids in!