No-one has cracked the golden driving style required to ensure trouble free motoring with the exhaust system
And by trouble free I mean reaching 21,000 miles or 2 years before the oil needs changing.
There are examples of people on here with all types of journey's, some have DPF trouble, others don't, everyone has dilution. The crux of the matter seems to be that passive regeneration of the DPF is rare, and the active cycles take longer than they should as it takes longer to get to temperature. This increases the chance of failed cycles, which in turn can mean the car spends more time in an active cycle injecting fuel until one completes which leads to increased dilution.
I have no evidence to back this up, but I suspect driving for economy will exacerbate the above as you won't be getting the engine into high rev ranges enough to get a quick warm up and also keep the exhaust gasses at a high enough temperature to get through an active cycle quickly. If you're an infuriating 56mph merchant trying desperately to reach the 53mpg mark using delicate glances of the throttle whilst dodging lorries then I wouldn't be surprised if you get more DPF issues....
Fortunately, I think the DS is quite good to actually drive and hustle down country roads, and it sits quite nicely at 80mph avoiding the lorry antagonisers when you're not on one of those horrid 'smart' motorways.
So my advice would be to enjoy driving it, it's much better than you would expect of an SUV IMO.
Servicing will need to be done at an authorised agent, and main dealers charge around £450 for a full service and £220 for an oil service. Authorised independents can do oil only services for less, but make sure they are authorised and can update the online service history. So, the service plan from a main dealer is usually cheaper than two full services, and also includes free Adblue top-ups if you're within easy visiting distance of one. Later MY cars get through more Adblue to meet stricter NOx requirements so that may be useful to you.