Now found a specialist trailer/towbar garage who did the job for £120!  He said BMW only ever supply the towbar wired for 10 pins as the Germans don't run their caravan fridges when on the move!  I thought it was a 13 pin EURO plug and Germany is part of the EU!

I have a 2011 model X3 30D with a factory fitted electrically retractable towbar. However, the 13 pin electric socket only has 10 of the pins wired meaning I am unable to run my caravan fridge whilst travelling. My local BMW dealer has quoted almost £600 to convert from 10 pin to 13 pin electrics. ...