It depends how you want it done.
One way is to replace the two 7 core cables back to the van with one 13 core, which would obviously involve the cost of the cable plus the socket and the time to do it. Probably looking at a couple of hours work
The other way, which does not "look" so neat is to use both existing cables, strip back the outer insulation about 6 inches and twist together to form one cable to enter the plug and wire up, in this case it is just the plug and time.Probably looking at about an hours work, plus the cost of the plug.
If you are handy with a screwdriver there is no reason why you sholuldnt do it yourself for just the cost of the plug and your time.
If you want to do it, just ask and I will post the wiring connections for you.