Hi
I have bought my chip at the citroendealer, but I know he bought it at carequipment in Belgium, nearby Antwerp. I life in Belgium so for me it was closeby.... www.carequipment.be or www.cituning.be But I'm sure there are chip in GB as well, surf the internet...
Your C3 really only needs 4.2 l/km? I have mine now 5 weeks, 8.000 km but he needs at least 4.8 l/km but I think that is already very good considering my driving style:-))
Ciao
Panino