fuel cap

I had recently visit the petrol station with the intention of purchasing some of Shells finest diesel. I walked up to the filler cap and tried to open it but nothing. I swiftly left the forecourt never returning. I returned home and read in my Merc handbook that there is a small round plate...

Hi all, I've searched through all the old posts, but can't find the same problem and therefore solution, so hopefully someone out there knows. a) I've a 93 C180, (and still have no idea what these W123 no's mean). b) I do not have any remote central locking ie No fancy button to press to lock...