I've just had a Thule one fitted. It's not detachable but with only 2 bolts you can take the ball off easily if you have to. "The Towbar Company" of Bournemouth came and did it on my drive. £325 inc VAT and they made an excellent job of it (but took most of the afternoon). This may have been partly due to the fact that one of them was a Thule techy guy who took digital photographs at every stage - apparently they wanted to put some 159 "how-to" information on their internal website!
Incidentally, I hadn't previously been aware that new towbars now all come with 13-pin electrical connectors (so you get a 7-pin to 13-pin converter included). This is to allow for trailers with reversing lights & fog lights. Unfortunately, German regulations say that if you have rear fogs switched ON on the trailer, they have to be OFF on the car, so the electrics are set up to switch off the car rear fogs if you are towing ... which is unfortunate if your trailer does not have rear fogs!