Some updates, but first of all I want to say thank you to everyone who took the time to read the original threat; reply; help; and even send me vicious emails.
We have a few charity shops in close proximity and yesterday DD offered to go and see if they had anything that would prove useful. She sent photos of two, one had fixed wheels and made of steel - DD said it was heavy.
The other was lightweight aluminium, with removable wheels, just needed a new leather/fabric seat as the current one showing normal wear-and-tear. Not a single blemish. I rang the shop to pay over-the-phone along with a nice tip. DD brought it home in a taxi.
It was only on examination, and enlisting Google, we realised this wheelchair retails between £250-£300, and I paid approximately half that, including tip. I managed to take out a loan (they call weekly and the interest is extortionate but planning to pay it off within six weeks anyway). The lady who donated the chair was using her Motability payment for a road scooter as her early-onset RA had started impacting her arms.
It's great to have what has felt like insurmountable pressure lift away, and will look towards a mechanical probably mid 2022, as this will serve us very well indeed.
So thank you, thank you, thank you for your time on my thread. 
(and an extra for some, you know who you are).