Nigel,
As part of the Volkswagen Group, (VW, Audi, Seat, Skoda & Mann truck) the parking aid sensor system is the same type fitted to all marques.
I have an Audi A4 avant 2.0 litre TDi with factory fitted detachable tow bar & rear parking aid sensors, I leave the tow ball attached at all times, it does not interfere with the operation of the parking aid sensors at all, either with or without the van hitched.
So the answer is...... fit what you like, fixed or detachable tow bar system.
On the question of nose weight, try looking in your handbook under towing a trailer, "accessories" thats where the info is in my handbook, BUT beware, in my handbook it states max tow hitch nose weight 75kg, but as my tow hitch is a factory fitted option, in the tailgate shut theres a sticker that states max nose weight is 80kg. Audi technical state that the upper figure is correct.