I'd look at the Volvo diesel range, or maybe a Mercedes C class diesel. You get don't have a timing belt with the Merc diesels so that's a bonus. A good indie keeps the costs down and they can do huge mileages in comfort.

Think the OP should decide roughly what size power and age of car is wanted first. We have this thread from 23.12 asking what car for long motorway journeys, then, inexplicably, 2 mins later another separate thread asking whether to get a 5 or 6 speed Captur.

For your distances, what’s wrong with a newer Mazda 6?

The newer Mazda 6 only comes as a saloon or estate and I have a preference for a hatchback, much more practical.

It's also a long car which is a pain for parking in town and for my employers rather small multi-storey car park.

I'm intending to keep the Mazda anyway as a second car and would probably look for a smaller one to do my main commutiing

For motorway miles traditional large saloons/hatchbacks i.e. Mondeo, Superb, i40 and something which may give more for your budget the Pug 508, with sensible profile tyres.

They are longer so give a good ride and being lower aren't affected by crosswinds.

with sensible profile tyres.

Very good point - when doing high miles/year this can become a significant cost if you get this wrong. Ultra low profile tyres don't last very long and can cost a fortune.

I have poverty profile 205/55 R16 Michelin Crossclimates at the moment on my Superb. Typically I've been getting 35k miles out of a set. Ride quality is better as well with higher profile tyres.

Plus the combo of low profile tyres and big alloy wheels are far more susceptible to kerbing and pothole damage, including wheel deformations, which can lead to either (more frequent) annoying punctures and (expensive) repairs/replacements or even blowouts on faster roads, which is very dangerous.

I can vouch for the better ride quality, having changed my wheels and tyres on my normally firmish riding Mazda3 from the OEM 205/55 R16 to compatible 195/65 R15 (CC+). It also takes more of the load off the suspension parts, which means they last longer as well, and the narrower tyres mean you get slightly better mpg.

TBH I only changed because the price difference in the alloys (nearly half as much for the OEM style) and tyres (about 25-30% cheaper) and both needed changing due to age/condition. Even more so if you compare standard (popular) sizes like both the 195/65 R15 or 205/55 R16 with those that are wider 18in ones - often half to a third of the price.