What towball and electric to fit

yes on my Thetford fridge the 12V will only work when the car's engine is running.

The old type single electrics wouldn’t normally power the fridge or the van's reversing lights so double (two socket) electrics were needed. These days people are tending to call the 13 pin socket single electrics even though the 13 pin replaces the old style double plug/socket.

Without having the old double, or the new style 13 pin plug & socket, the fridge will not cool on the move and reversing lights on the van will not work. 

The leisure battery doesn’t normally power the cooling element of the fridge, only the control panel and, maybe, the fridge's internal light. Flicking the switch for the fridge to 12v enables the fridge to run from the vehicle battery once the engine is running and not from the leisure battery. In order for the fridge to run from the vehicle battery, the necessary wiring connection needs to be in place - either the old double/twin socket or the newer 13 pin connector.

you're far more knowledgeable than me TW but even with the newer 13 pin socket I had to have an addition (labelled supplemental) wiring harness installed to enable the 12V feed for charger and fridge.

Not such excellent advice as far as I can see. Apart from not powering the fridge and leisure battery charging en route, as other people have pointed out how are your reversing lights powered if you only have a 12n socket. Assuming your van has reversing lights which have been fitted for many years now, it is a legal requirement that they work.

Thanks, Corners.

As I understand it, most modern vehicles need a supplementary kit to match British caravan wiring requirements to those of the computerised electronics of the towing vehicle. 

OK guys let's simplify this.  A 2012 caravan will come with a 13 pin socket so why bother with anything except a 13 pin socket on the car.  Type of hook (detaceable, swan neck or fixed) depends on  OP's preferences and budget.  All towbars sold in UK have to meet EU spec so again choice is up to OP.  If you get a dedicated wiring it should power up all, but do check with fitter.  I have a VW Tiguan with dedicated wiring and everything works. I sometimes tow a trailor so have an adaptor to allow 7pin on trailor to fit 13 pin on car.  .Before a trip I put the fridge on mains for 24 hours to cool it down (and charge battery up);  the fridge will only work on 12v if the car is running.  Robidoo, it really is quite simple so please do not get confused.  I had my towbar fitted by a local fitter and am very satisfied with quality and price.

Actually, I think we have explained things quite simply, Comey.

One poster seemed to have been given less than ideal info and I think we've explained the situation for him as well as the OP.

I have just swapped my Cherokee with a witter detachable to a grand Cherokee with a westfalia detachable and must say westfalia much more simple to fit and remove. even the protective cover is better.

Worth noting that although all tow-bars have to meet EU standards - there are sizing differences among the various manufacturers - so I personally wouldn't just assume that all tow bars will fit your tow car with the intention of towing a caravan.

Audi's factory fit detachable swan neck for example is a Westfalia bar which doesn't give enough clearance off the rear bumper on A4 S-Line models for the Alko hitch. We went for a detachable swan neck from Tow-Trust that had a longer reach. Our electrics were installed by Audi so cant comment on those - but lead to believe there is some coding to be done for the ESP, Parking Sensors etcs. 

Our Kia fitted electrics automatically shuts off the rear parking sensors when plugged in. Clever!