Thanks for the reply, this is what the supplier's web site has to say, which tbf I took to mean this was needed for my car, but their own fitter says not. I just thought I'd check before going back and insisting they use the correct harness.
" This original type harness option (sometimes called a specific harness) offers a tailor-made solution for your car model.
The electrical configuration is 100% preserved by managing the radars and reversing camera as well as the various driving aids (ESP, emergency braking, etc.), which the universal harness cannot manage.
If your vehicle is equipped with this type of driving aid, we recommend this option. In certain cases (for example for vehicles equipped with emergency braking in reverse), this option may be made mandatory to prevent the detection of the ball from putting the vehicle at fault.
This type of harness is connected to the vehicle's original connectors. However, it may require programming using a diagnostic tool. If necessary, programming of the harness is included in this package. "
I'm not too worried about trailer sway tbh, it'll only be pulling a small motorbike trailer anyway, but I've towed a lot of stuff of all sizes over the years ( HGV class 1 ). I'll cope

If speed kills, buy a 2CV and live forever.