Sounds like you're setting about a worthwhile project Carl. It seems as if there has never been a standard system fitted to Dandys/Rivas, at least not a system as you would find on a caravan. Our first Designer from the early nineties had a 230v system fitted and the 12v system was a cobbled together affair with wiring running through from a leisure battery in the gas box to inside where a loose connector allowed the use of one or two 12v lights only (loose ones clipped onto roof bars or extended through to the awning if a light was needed in there.) The sink tap was powered by its own battery pack.
Our Destiny from the late nineties had the benefit of a charger/tranformer which meant that the sink tap and internal 12v fitted lights, as well as the 12v system on the fridge would work on 230v Electric Hook up. The fridge would also work on 12v when towing. We were assured when we collected it from Dandy that the charger/transformer that had been fitted did not need a leisure battery fitted for the system to work smoothly. As a result of this we never used a leisure battey and always had an EHU wherever we went.
Our current 2007 Designer is another variation on the theme. A good quality charger/transformer has been fitted which will allow the fitted 12v sockets to be used from an EHU and will also charge any leisure battery that's being used, just like our previous Destiny. This is fine and dandy (no pun intended) but of course you don't need a leisure battery if your on an EHU. The difference in this case is that there is an absence of a connection to the car. There is ancilliary towing wiring (The grey cable and plug) situated in the gas box. This is connected to a leisure battery by a short lead with a suitable loose socket, to take the grey cable plug, on one end and two bulldog clips on the other to connect to the battery. Tracing the grey cable to inside the trailer leads to a terminal block where all the different wires in the grey cable are separated and secured but only the green and white wires are utilised by being connected through the terminal block, to the 12v sockets and to the charger/transformer. This means that although there is a working 12v system off the battery there is no means of re-charging the battery from the vehicle or to actually connect the battery to the vehicle. Not a good thing if you like utilising nice cheap Certified Locations that don't have EHUs for more than a couple of days (And we do!) or if you have something like a Propex heater which needs gas and 12v to work.
I did query this with Ian at Riva who agreed with what I said and suggested that the only way round this that he was aware of was to fit a split charger to your vehicle and then wire the trailer up so that the leisure battery would be charged when the vehicle was running.
To date, through an electrician friend, I've discovered a firm called BROCOTTS who advertise on Ebay and sell a self switching split charge relay kit (12v/30amp) for about £17. I'm assured that this is exactly what I need for the car. I just haven't yet sorted out the connectivity from car to trailer. It can't be that hard though, Can it?