Hi all - help appreciated. I have recently acquired a 2019 A6 40 TDi Auto and have noticed a few times when going downhill the car gearbox changes down and the revs rise to over 3000rpm. Is this normal for this car or something that needs investigated?

It's normal. Some VAG automatic gearboxes will enforce a lower ratio if the car detects the speed is increasing despite having the brakes applied. Personally I dislike the system as it forces high engine revs to little effect on decelleration. There should be a switch to disable the feature.