You're not really £18000 down. You're down £4000 for the repair plus the difference in value between when you bought the car and now, which we all are in the same boat as you when owning a car! If it was me here's what I would do. Have the car repaired as you are and then sell it immediately to an online bulk buyer, it will then go to auction and that's your involvement finished. I'm guessing you'll raise maybe £12k ish? Anyway, go and buy something for that value max, a petrol estate such as a Toyota auris, something bombproof. Buy from a main dealer, you'll get 12m warranty standard, which will be extended yearly if you service it with them. For sure it's bad luck, but you unfortunately bought the wrong car for your useage and one with a bad reputation as well. Good luck, don't let a car ruin your life!

which causes the issue to develop in the first engine.

But is it the first engine, we don't know? It's a seven year old car with 50K on the clock. IME, folk often run diesels up to very high mileages before getting shot - unless they experience problems. Perhaps a previous owner did what the OP's being advised to do here?

That's what I was saying - if predominantly used for longer trips (car is warmed up and thus only needs to passively regenate the DPF), especially on free-flowing roads, they shouldn't have a problem, and probably why Mazda dealerships aren't choc-a-block full of cars with 2.2TDs with serious problems (i.e. all of them).

The design fault means, in my view, that they are far more suspceptible to DPF and the oil dilution problems, but all modern diesels are, just to lesser degrees. Even with petrol engined cars (without PPFs) you can have problems associated with repeated low mileage trips from cold (noit just battery degridation). My old 90s 1L petrol Micra hated that sort of 'shopping car' treament, due to the car's engine sensor design.

The problem with the mazda 2.2TD design ssems to me that once the issue starts, the only remedy is to replace the engine oil, and that could happen quite often, because no car I know of has a warning light (like some HGVs may do [sensibly]) that an active regen will be needed soon / now, to enable the driver to take action to facilitate it without damaging the oil or DPF.

A manual DPF active regen button would be useful, only enabled when that warning (say 'next 100 miles' or whatever) appears on the dash. Having such a facility would've saved owner a fortune in forced regensm replaced DPFs, extra oil changes and other repairs associated with oil dilution.