Thanks @Mossfinn I’m glad that you were told exactly the same as me.
Motability have to use the term ‘for the disabled persons benefit’ in order to comply with the VAT exemption and to give them grounds to recover cars where there is clearly abuse. The example they gave me when I had the discussion was someone like a grandchild or niece/ nephew who lives elsewhere being in possession of the recipients car and using it as their own car, rarely, if ever, using it for the benefit of the recipient.
Clearly in a situation where a couple live together and only need one car (as, in my case, my wife can no longer drive), there are going to be times when the partner uses the car in a way that is not directly for the benefit of the recipient. It would be daft to assume otherwise. However, as you say, if my wife calls to say she has fallen or needs me, I need to be able to get home quickly. Also, my wife would probably argue that all times that I spend away from her are directly for her benefit 🤣