It pays to shop around, I did just that when CDF quoted me originally as it was an additional £100 more than I had paid the time before. But other prices were astronomical and they certainly ended up being the cheapest by far. I think its a Witter tow bar but not entirely sure. There's not much info on their website.
My particular car has reversing sensor but it also has a button on the dash that you can press to temporary stop the reversing sensors from working thus making the noise, but I'm not sure if all cars have this format. And like you say it only comes on when you reverse which isn't very often in the scheme of things although I do admit its a bit of a distraction when it does happen. I used to have a plate on the tow bar of the previous car but not on this one, I could do with knocking one up and getting my mechanic to fit it to save the body work in case of accident! Its extremely annoying when you misjudge the distance and cars these days have body work which is so fragile, I have one of those electric handbrakes that mean hitching up can be a bit of a nightmare as he car does tend to leap back a small distance when you reverse and go past the 'biting point' on the clutch (unless of course you then override the electric handbrake) which I've not managed to figure out how to do cleanly yet.