It can be anything really.

You need to read the fault codes to get some guidance.

Your engine historically has a few design faults resulting in stretched timing chain evry so often and leaking fuel injectors that lead limited oil circulation, destroying the turbo and the engine.

As there is already damage to the engine, it will be best to read the codes and not to try to push the car with the hope that it will clear out.

You can read the codes with Forscan:

https://www.google.com/search?q=forscan&ie=utf-8&oe=utf-8&client=firefox-b

A turbo specialist can fix that but before you fix it, you must find the source of the problem, checking the chain via Forscan and the injectors and oil pick up.

It seems that you are in for a big bill but if you had the car some years and plan to keep it, the overall cost averages low over the years. If you just bought the car from an auction dealer, then there is a good chance that the previous owner had the problem and traded it in.

It seems that on this website they have the details and on the personalised one with the registration they have a recall note.

Just print out the recall you found and show it to the dealer.

Is this what you use to check the recalls?

http://www.dft.gov.uk/vosa/apps/recalls/default.asp

There is no 259 recall there.

I have the second generation car and when I got it had done less than 15K miles in 3 years. It was regenerating a lot but gradually it cleaned up. In that sense, if you drive in traffic all the time, the car can only clean up that much on a journey.

The only way to keep an eye on the consumption is to keep long term statistics. I bought it, went for a run around the corner (~100 miles) and before I returned home, I filled up and zeroed the counter. Since then I fill up and add the miles and the litres. I use the same website each time to give me the figures for that fill up and the overall. In that way, the current fill up may be not be good depending on traffic, etc, but the overall mpg is absorbing that. While the trip computer is still increasing the mpg, it is lagging behind the real statistics by 10+ mpg. At some point it will match up.

When I bought the car, it had the oil changed but as it was regenerating a lot, it started rising up as you say within the first 2K miles. They removed a bit of oil but later I realised this was down to the regeneration. Now I am due a service after 13.5K miles since the last change and the oil level is half way through.

You should go back with the invisible recall and ask them to do it properly and make a note that you are not happy with the software update. If the car is not running wrongly, the consumption statistics after the update may not be great but they may have been like that even before the update.

A lot of garages do failing parts only and they do not touch anything else, e.g. replace one injector seal, one shock absorber, one spring, one calliper, one bearing, etc. These should be done in pairs at least for the same axle. So for injector seals, if you have found out that these needs to be done, do all of them. There are old posts about a Mazda specialist called AS that they advertise that they do the seals for £95. So if Mazda has a recall for it and they just do one of them, it sounds that they play very cheap as the total cost of doing them all is almost nothing.

https://www.youtube.com/watch?v=2FzfrdoMUaI

If the bulb needs replacing, you do not need to replace the whole switch. It is just a bad bulb. So you replace that. I am not sure if you want to replace the switches for some kind of cosmetic upgrade though.

If you decide to replace the bulb, you have to remove the plastics to get access to the bulbs. You swapped them with something else and it should work. In the 6 you can remove the plastics without using any tools. From these photos:

http://w7dps.com/mazda/console_removal.htm

Pull the cup holder like here:

https://www.youtube.com/watch?v=tSyb0VzGMnY

If it goes out, just use your finger to push out the other plastic with the switches.

The bulbs should be something like this:

https://forum.mazda6club.com/electrical-hvac/333121-heated-seat-button-bulb-size.html

Quite possibly your only chance of getting one new is from a dealer. But if you are looking for a used one, a scrap dealer will have one. They have web sites that you describe what you want and they come back with a price. But check with the dealer as well as a new part may cost less.