Assuming N type socket, you need STC50081 for the genuine LR part.
Fitting is simplicity itself, there's a socket behind the drivers side rear light cluster, you plug into that multiplug, route the cable, and it's done
Put STC50081 into Google - should be around £40.
Just out of interest, how is the £40 genuine part different from a £8.99 socket off of eBay (for example: MAYPOLE 12N PRE-WIRED 7 PIN SOCKET TRAILER CONNECTOR on eBay (end time 26-Feb-11 09:26:53 GMT))? £31 for the connector to plug into the car electrics easily?
Seems a bit of a rip off to me....