The dealer has told me it can't be done as the wiring is not set up for it. The tech's advice was to get one of the aftermarket guys who will come and fit for you to do it - not sure about that as unless they are familiar with the car (which they won't be), they won't know where to drill holes in the bumper and I don't want any 'trial' attempts that meet metal 5mm in!
So, I've ordered a set of good flush mount sensors that simply work off the feed to the reversing light. Now - anyone know how the rear bumper comes off?