Interesting. Would you say that buying a used 'modern' diesel just prior to dpf introduction will have similar low reliability/high long-term running cost issues?
Or is it the dpf that makes such a huge difference?
DPFs are definitely a large, but by no means the only, factor. Especially once the MOT test gets its act together and properly checks for DPF removal (the current diesel emissions test is, frankly, a joke for a modern diesel given the health concerns now being aired). External visual inspection for the presence of a DPF, whilst tripping up a far few "early adopters" of DPF deletes is similarly limited.
Consider that prior to returning to Jap ownership a family member had a 2005 diesel Merc (non-DPF) diagnosed as needing a new engine, and Merc's response when approached for a contribution was "120k miles isn't bad for a diesel engine".
As I explain (also time and time again) when someone "calls me out" for suggesting that modern diesels are unreliable: It's not so much that they're necessarily unreliable on a day to day basis, it's that a modern diesel has a turbo, DPF, DMF and very, very high pressure injectors that a modern N/A petrol doesn't and in exchange has very little advantage (for most people, hyper milers and BIK tax avoiders excluded) when everything is considered over said petrol. None of those things are cheap to replace when they inevitably go wrong. 100k seems to be the magical figure.
How much better MPG do you have to get to balance out a DMF (usually about a grand, all in) replacement alone and the greater cost to buy and fuel a diesel?
As to pre-DPF cars: I think DPF's really started to come in around 2005-2007. Worth considering that most diesels of that age will be getting "leggy" by now. I'm reasonably sure by then that common rail injection systems were the norm. I'd still buy petrol in that age range-turbos may well be creaking and DMFs were also around.
TL;DR: All factors considered, you have to be lucky to make a diesel work financially in the UK, unless you're into BIK tax issues.