Hello Rod,
Having a Series 3 diesel that I wanted to upgrade to a dual circuit system with a booster I have been down this road myself. You have two options. 1) buying a Peugeot vacuum pump that drives off a pulley from the fan belt. or 2) get a 4DBI or similar Isuzu alternator with a vacuum pump fitted to it.
There is a member here who wrote a thread on the Peugeot option - the part is also the same as an English car.
I decided to go the Isuzu way - then the clutch died more than just the hydraulics and I bought another Land Rover that was in better condition.
I will try to find the thread. It might have been a blue camper from memory. Just checked it was a member by the name of "Phin" I replied to him in 2014 and that is the last time Phin posted .....
The thread is under the subject Series III Dutch Camper and the part about the Peugeot pump starts at this link
https://www.aulro.com/afvb/series-ii...ml#post1838007
I found it was a cheaper option to go for the Isuzu alternator with the vacuum pump.
Oh BTW the later Series III diesel - a bit after 1976 did have a butterfly valve integrated into the system that provided vacuum for the booster. However, they wore out and become unreliable. Now it would be extremely hard to buy parts for.
Kind Regards
Lionel