Cheers Crimguy - thanks for the advice. I was quoted just over a grand for mini to put the sensors in for the back (found the notes I took at the time) - which I thought was excessive, but not surprisingly so. I haven't even got the car yet, so I'm jumping the gun here. I have a space RAC parking kit, so once I've played for a few months I have a feeling I'll be installing that - by which time I reckon there will be a few people who've done the same.
As for that 20 second wait - I would imaging that quickly becomes a PTA.