My daily commute takes me over the Severn crossing and the Yaris sometimes get pummeled fair bit with the cross winds and HGV's roaring beside you. So wanting something a bit heavier to deal with stability and more BHP to get away from the HGV's.

Is the Leon 1.4 TSI is reliable as say a an Auris? I usually keep my cars for around a decade before shifting to something younger

As I said, the Leon is fine if you stay away from ones with all the toys, hence the SE 140 suggestion. The petrol versions of the Japanese cars will still be more reliable in the longer term, but with a decent service and MOT history, you probably wouldn't have too much to worry about.

One of the keys to having a stress-free onwership is also to find a high quality mechanic, whether an independent, indie specialist or main dealer. It's far easier to with the likes of Toyota/Lexus and Honda, much less for with Mazda, VAG and Ford, where the main dealers can be highly variable in post-sales quality.

I own a gen-1 Mazda3 1.6 N/A petrol. I've owned it from new for 14 years now and, touch wood, it's had no serious issues, just general wear and tear replacement parts. It still drives very well even if its not considered 'quick'. The gen-3 car is a step up, but not as much as to the VAG 1.4 TSI (not the terrible twincharger 160PS unit) 140/150PS unit (though the 122PS unit is still better to drive than the 120PS Mazda 2.0).

The gen-3 Mazda3 is a good car, and I'd go for the best value one which is the 2.0 petrol SE-L (with or without Nav is up to you - only the run-out ones [above your price range] have Android Auto/Apple Car Play), which has a decent spec but is shod on sensible 16in rims with 60 profile tyres, which means the ride is comfortable and the tyres will last and won't break the bank when it comes to replacing them (unlike the Sport models shod on 18in rims).

Again, as SLO says, the service and MOT history will indicate whether whatever car you look at has been looked after or not. Make sure such cars have the correct level of service throughout their life, including all the correct spec fluids and filter changes at the right time/distance, as skipping them on a regular basis indicates owners who neglect their car.

The same goes for regular MOT fails on items the owner should always be keeping an eye on, such as defective wiper blades, bald or unevenly worn tyres (including use of ditchfinders). The same goes for cars with very 'well used' interiors. Minor items (but not failures) such as small scratches etc (that haven't rusted badly) or a small ding not affecting anything is fine and would be both par for the course for an older car.