Probably a plug in hybrid like a Hyundai Ioniq or Kia Xceed (there are others, particularly from Kia, Hyundai, Toyota, Mini, BMW, Jeep).

If you don't want to plug in, standard hybrids (Prius and the other kind of Ioniq among others).

If you have an aversion to electricity, a small capacity turbocharged engine. Fiat Twinair is the best known, but most manufacturers now provide little turbo engines in most of their range.

What car is best on fuel for short journeys, that has some of the comforts of a medium family car? Do all cars suffer poor economy on short journeys? The official figure urban for ours is 45mpg and we struggle to get half of that

I'd stick to the diagnosis thread otherwise you could be at crossed puproses. Wait to see what the problem is first before diving in and changing out the car. No way should it be getting that low an mpg unless you're tooling around at high revs or there's some (as yet undiagnosed) problem with the car.

My gen-1 car never has gone below 34mpg on heavy traffic trips.