Many thanks, DavidRon, as an 'owner' you are not expected to be across a system that is a lifetime fill for many examples and usually not needing attention for 100,000 miles or more. Simply adding fluid will not be enough as the car has no way of knowing the actual level of Eolys in the tank and this is why the ECU must be given the initial fill quantity. The rigid tank in your C5 is to the left of the fuel tank under the LHR passenger position - access from under car and under an aluminium protector shield.