Topped the additive one my wife's Pug about 4 months ago, It cost (retail) £110?? for 3L complete with filling kit. That was at K98.5miles. I tried to fill it up before the warning light came on but was not quick enough. It really was quite easy.
A guide here
http://www.glodark.com/peugeot-diesel.htm
Not sure which additive is used by later Bingos but there are different types for earlier models. My wife's was green connector, see pictures 2 & 3 in the link. The connector colours match up to the additive type. Others include white and ?? Don't guess, double check and some are not interchangeable. I think they're all using green connectors??
Green connecotr is EOLYS 176 also known as Infineum F7995.
There are "alternatives" out there and I don't know if they're any good.
If you leave it too late to top up then you need Peugeot Planet 2000 or similar OBD2 software to re-set the error code. In my case it would not re-set as there was a fuel filler cap error. Yes, every time you take the fuel cap off it registers and injects a small amount of additive into the fuel. (full tank every time to get the most mileage out of your additive) The additive is also used the force a regeneration (not sure of mileage when it does it) by injecting in the exhaust?? I think that's how it's done. This is normally triggered by highish revs over five+ miles. Hence these DPF cars are not great for city users. The regeneration causes the PDF to cook or bake up to around 450 degrees burning off any diesel particles.
Eventually, after 100s or 1000s of regenerations the filter becomes too clogged and needs cleaning. Don't got to a dealer. There are companies out there who do this job, come to your house and remove and clean the DFP for around £130??? Again, there are numerous guides on tinternet.
Some have removed the filter and then had the CPU re-mapped to get past emmisions tests...... this is another complicated story.
I believe the 90BHP MK3s, pre-2012 delivery are Euro 4 and don't have a DPF. (which is why I bought mine to avoid problems) Post 2012 are Euro 5 and have a DPF as I understand it. Not sure about other models but I think most do have a DPF.