Volvo penta d4 dead, stone cold dead.

Is there any way of diagnosing whether it's a failing hcu while it's in the intermittent stage without waiting for it to fail permanently?

There was some moisture in the connector block that provides power to the EVC system.
You could remove it and have it bench-tested by a Volvo specialist. Or, try to find a good unit to borrow, plug it in and try to start the engine.
 
Also, those units are not “repairable”. Once they start to fail intermittently, you need to replace them. Do you have an authorized Volvo tech in your area? A VODA diagnostics would give you more info.
 
So to (hopefully) close this one off.

The power trim fault was caused by moisture in the electrical connector between the relays and the trim motor.

And the completely dead thing was caused by moisture in the electrical connector that plugs into the side of the engine above the air filter that feeds power to the EVC system.

A Liberal spray of contact cleaner has fixed them both, I will keep an eye of things for the next few runs out.

I suspect it was caused by the valeter that I asked last week to clean up the bilge and engine bay in prep for selling the boat being a bit too judicious with the water.
 
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