I'd echo ChrisP's advice regarding the 13-pin socket on your tow-bar. It's a much better made and more reliable connector compared with the older 7-pin standard.
Your Dandy when it finds you will most likely have a 7-pin plug. Ideally you should get someone to replace this with a 13-pin type but in the meantime you can use a simple adapter like this - [You must be registered and logged in to see this link.]
While getting the 13-pin socket on the car is a good idea, you should check with your fitter the relative cost of having it fully wired up, or just wired for the seven basic lighting connections.
Some cars have all the necessary wiring brought to the rear and the 13-pin trailer kit will be just a few pounds more expensive than the 7-pin one. However as the automotive industry is obsessed with penny pinching many cars have the internal wiring omitted and the fitter would need to install wiring for the fridge and battery charging connections through to the battery location, which is a time consuming and costly exercise. There is no point in paying for this if you only need the lighting connections for the smaller Dandy.