Sorry Kat just re-read your post and when you said “hand controls” I took as a device for controlling the cars electrical systems, in my case remote from the steering wheel via a hand steering ball.
If you only had the car since last week I think Motability needed to be called if the dealer is saying they can’t find anything wrong with the car and are passing the buck to mechanical hand controls that have no relation to the cars electronics.
Motability will give the dealer a call on your behalf and see what the problem is and how to go forward from here in either rejecting the car or giving you a hirer car with the adaption you need until they fix your current car.
This should not be happening on a car just out of the stables and needs immediate attention on the dealers side.