I believe you need 3 litres if its empty you may also require the citroen computer to tell the ecu that it has been refilled and also that the tank cap is closed.

As someone has pointed out, unless you have access to the Citroën Lexia computer or equivilant you are wasting your time, the car calculates how much Eolys it has used and then throws an emission fault when it thinks it has run out. Citroën are going to charge you an hour's labour so it's going to cost £100 before you've bought the fluid.