@CarpeDiem thanks for your thoughtful and detailed answer, I realise reading you that this part of my electrical refit is more complex than anticipated and that I haven't done enough research.
It seems like there is no way around a "hack" to install the Mastervolt external regulator, which feels a bit silly.
I did not know about the Nordkyn product which seems to be purposely designed around the constraints we face on a boat set up, wish I knew about it before.
I guess I have modified my alternator already, so backing off now would be a rather costly exercise involving buying a