Kia Venga

There's no easy answer to this, because what's comfortable for one person is uncomfortable for another. You need to try various cars in the showrooms and see what works and what doesn't.

Volvos are generally agreed to have some of the best seats in the business, but most are probably bigger that she'd want, coming from a Yaris. The C30 might be worth a look although this is a a rather low-slung coupe. As suggested, something higher off the ground like a Kia Venga or Nissan Note will be possibilities.

Another Yaris?

I also have back issues although mine usually relate to lower back. I used to love Saab seats but clearly that is no longer possible. Volvo seats are my next favorite. Some VAG seats are reasonable but not all...

My ol' Mum went for a FIAT 500L. Finds it very comfy - just travelled from Wales to Toulouse and back in it. Easy access and egress due to the body shape. 1.3 diesel very economical and peppy enough on the motorway.

Amazingly I have found the seats on the Kia Ceed really supportive particularly on long journeys; not sure if this applies to all trim levels, try doing some test drives at a dealer near you. I really notice the difference when I have to drive other cars, and particularly the work VW Caddy vans which have dreadful seats.

Everyone's back pain is different although upper back and neck pain may relate to driving position rather than the car. I have found a more upright car to make life easier in this regard but you need to try different models. I have driven the Suzuki Splash and it has an excellent driving position although how good it is for hours I cannot say. Find a car with a fully adjustable steering wheel (not just in and out but up and down) and height adjustable seats and you should be able to obtain relief.