As an ex Prius owner I'd avoid hybrids. Ironically for an eco car they like to be well used or the batteries quickly cause problems.
I wouldn't write off the idea of an electric car. Might be worth a chat with you local council to see where they stand about marking out a disabled bay outside your house and providing charging points. I wouldn't hold your breath, but it's gotta be worth a phone call or email.
Avoid a modern diesel with the mileage you do, they like a good bit of heat through the block before the give their best economy, and DPFs and the like will give you a headache on a low mileage car.
Which leaves petrol. Something small and probably 3 pot. I don't know anything about your good ladies disability and access needs, but that not with standing i'd probably start my search with something like the Citroen C1. The Fiesta is a good car, but not as economical in petrol form in the real world, and probably a touch bigger than necessary for a sole person to scoot locally.
As above, I'd be looking at something older. As a random example, I've just picked up a mint 1 owner 60k FSH Fusion for £700 for my own local pootling. It's doing mid 40's MPG, so I financial terms it's probably on a par with spending £9000 or more on a new car that does 400 MPG or something.
One final thought - if she's on the higher tier mobility element of P.I.P. she would qualify for a motability car like Mrs Drago does.