Definitely not a steel washer.Use the correct copper washer and get it from a Peugeot supplier .It needs to be an accurate size.You may not need a cutter kit,but it is important that the mating surface is perfectly clean without any debris.Removal of the injectors on these is not an easy job,as various other parts need to be removed first for access.

You must cut the seat, and only use the correct copper washer. They can be a real pain to get out - even when they are chuffing; they can just turn up their toes when you put them back in; and it's very easy to break off the retaining studs when you tighten them back up. I farmed mine out to a garage! On my 307 all 4 came out and the seals were done; on my Mrs C4 Picasso, one refused to come out, and another refused to work when it was re-installed so had to be replaced. Not a fun job....