Thanks to Gordon, Manlio and others for their helful suggestions and
reports.

I've determined that my HF ground does NOT connect to the engine or
other underwater fittings, only to the chainplates etc.

And that one sistership also reports accelerated erosion of her prop
anode when making passage, but not when in dock. And that sistership
has a capacitor-bridged break in the radio counterpoise, as per the
Stan Honey recommendation in the article noted by Gordon. The
sistership also uses her HF Pactor modem extensively and does not
spend much time connected to shore power (far less than I do).

I've purchased a silver/silver chloride half cell and aim to spend a
few hours following the Nigel Calder test suite detailed in his
article at http://www.proboat-digital.com/proboat/200604/.

Any other tips?

Cheers

Bil