Thank you for the reply you have saved me a few quid.
I got a genuine Citroen wiring kit from my local dealer for around £35. It was for my Mk2 2006 Enterprise van. It fitted straight onto the connections at the rear light clusters. One to each side then run the bare wires on the ends to where the lighting socket is on the towbar. No requirement to interfere with the BSI unit or any other electronics. Only thing that I was not happy with was that there was no indication either by sound or warning light that I had a trailer connected. By law there must be some indication. Dealers did not want to know/ not interested. I bought a buzzer from the local trailer spares outlet for £4 and fitted it to the indicator wires at the rear. Problem solved. Foglight on the van disconnects when the trailer is connected as well.
Dedicated wiring kits normally have speeded up indicator clicking to show indicator bulb failure. This is apparently legal & audible warning is not needed.