wot peugeot's depolution additive called

l do not know but l know what is spelled WHAT!

It varies according to age.
The earlier fuel additive was called DPX42
The later stuff is called EOLYS176.
l think the stuff is almost £20 per litre. You will need 5 litres unless ur car is 55 plate or newer in which case u need 3 litres.
This stuff should only need to be filled at certain services (depending on age the service intervals varied) in which case u must replace the Particulate filter (FAP). A new FAP is £500 with a £400 refund when u return the old FAP. If u do not replace the FAP u will get anti pollution warning messages until u do as the differential pressure sensor will be able to tell how loaded the FAP is by monitoring the different exhaust back pressures.

The only other reason that u should need to top up this fluid is if the car has used it all before it should have done, in which case u have a problem. If the additive injector has stuck open then it could have injected all the additive into the fuel, which will clog the FAP, which will cause the car to undergo an excessive amount of FAP regeneration, which could cause it to post inject so much that the sump starts to fill with diesel causing catastrophic engine failure. l have seen this happen before, more than once.
If u think that u r using too much additive then let a dealer take a look at it for you, it could save u a potential repair bill of about £5500