I tested the charge rate on the van yesterday - good job i did!

At idle, it was sitting about 14v and when revved it reached 15.9v before I chickened out.

I tried two of the 'solid state' (three prong) type voltage regulator but neither helped. In the end, we fitted an old 'coiled wire' (twin prong - earth via bulkhead mount) type from a Diane.

The older type have an adjuster inside so you can set the rate of charge. I should have taken photos

Essentially, you turn a wheel which adjusts the regulator so it outputs more, or less volts.

Hetty is now idling at about 12.5v and when revved goes up to a more sensible 14v.

I'm glad I could cure it this way because (typically) the alternator is a weirdo 60s 2cv one that citroen only fitted specifically to 350s for about 6 months of production

Some questions:
Why were the two solid state regulators not up to the task?
Am I going to blow up my phone/stereo/lights/battery if the battery is holding 13v instead of 12v?
Can I buy any little plug in thing that will further regulate the power supply so that only 12v is fed to the CD player/12v socket, or does it not matter?

I'd guess that the earth lead on the 3 pin plug was not making contact, or isn't actually connected to earth, since electronic voltage regulators are usually directly interchangeable with the electro-mechanical type.

IAs far as I know, the only difference with those alternators is that they have a couple of 'prongs' poking out of the back, which are connected to the unrectified part of the alternator's wiring.
That's used to operate a 'lack of charge' warning light, via a small box of electronics on the bulkhead.