Even though I'm not exactly a fan of diesels, if you do more than 20-25k miles pa then diesels should be fine, as long as they are not all town driving/short journeys. If its all town driving (jig-jog traffic), normally I'd recommend either a small petrol or hybrid, but given your mileage and budget, they wouldn't be suitable.

My guess is that you do reasonable length journeys, so a diesel should be fine - most of the problems associated with them come with using them on short journeys of less than 10 miles/15 minutes running.

The Mazda 6 is quite lareg compared to the i30 - do you need something as big as a Mondeo? You may want to consider the Mazda 3, which comes in both 1.6 and 2.2 TD guises, or similar cars from Toyota, Honda etc if you wish to pick from Japanese only. The i30 does have diesels available as well. If you're looking for a car with useful load space, then go for the Focus-sized estate cars (no Mazda 3 though), as they're likely to still be cheaper than the hatch Mondeo-sized ones, plus the flat back may help in parking.

I have a Mazda 3 saloon, which does come with 400 litres of boot space (abot 350 in the hatch with the parcel shelf in), but has a small boot aperture (difficult to put large cube-shaped boxes in, but fine for suitcases) and can be a bit tricky to reverse-park without sensors due to the high boot line. A shame as i think it looks much nicer than the hatch. The 6 is a good-looking car in all guises (even the early models do as well).

Also considering a petrol Avensis but worried about mpg.

On long runs you'll get an easy 40mpg or more with a 1.8vvt. They are very good.