The diesel adative required is Eolys, there are 2 different types so you'll have to check you get the correct one. Add-blue is a totally different system.

When I had a Citroen with this system I topped up the Eolys but didn't bother to get the system reset. It still pumped the fluid but gave the warning message at each start. It carried on working like that for the following year I had the car.

Usually when the fluid needs replenishing it is also time to clean the DPF, this will eventually need removing & jet-washing clean, or if your pockets are deep get an exchange one fitted.

If the Eolys runs out the DPF will block & engine will be in limp mode & eventually undriveable

scrap that, ive just read this;

http://c-crosser.org/plugins/faq/faq.php?0.cat.1.8

completely different to add-blue systems and im guessing if i dont fill it up the cat or particulate filter will just fill up with soot and choke the engine aswell as put loads of fault codes up?

Yes,there's two types of "ELOYs" fluid,what you need depends on the colour of the filler cap connection,black/white or black/green.they're not interchangable,you put in the wrong one and you'll need the other addative ECU as well.

I buy my fluid from my local motor factors,Peug's quoting £45/L and today's one took 4.5L to fill it up,factors price was £190 for black/green cap,£145 for black/white cap for 4.5L

Run the reset and job done,as long as the DPF filter in the exhaust isn't blocked of course..