hello graham---look for the simple first, bulbs and fuses, then earth connections --- the front lights in particular-----the fact that both have gone on the one side I would be looking at the fuses in the van first, if they are ok then the earth connections (white wire)----clean them up---the coupling plug may have got damp into it, or the pins need cleaning, if you open it up and find that any of the copper wires have turned a dull brown, you will have to cut the wires back till you get good copper,if you feel you are not up to doing this, then find an electrician, I wouldn't think it will cost you a lot
bob