13 way cable and things available form Vehicle Wiring Products tel. 01159305454 - unfortunately no website.
I've converted my van to 13 pin by making up a lead about a metre long with 13 pin plug on one end and 12S and 12N sockets on the other. The caravan plug leads are unthreaded from the drawbar, plugs cleaned and then mated with the sockets which are then wound with self amalgamating tape. leads are then coiled uo and the coils fixed up under the front of the van with cable ties while the 13 way lead is fed back along the drawbar and again fastened in with cable ties.
This way you goet all the benfits of 13 pin but the van can still be towed (eg by recovery vehicle) by simply snipping the cable ties and using the original leads. Also a good point when you come to PE or sell the van as most customers / dealers will still prefer to have the older system.
Finally a 13 pin connection may deter the odd rogure, as not too many of them will have the right socket on the thieving vehicle.