Hi Leon and welcome to the club. I'm guessing that the bit of plastic that connects to the flat metal springs has broken. I have just been out to my s type on the drive and had a good look, and see that it can wear. Mine does lock when the car is unlocked --- maybe I have not allowed for enough delay, or maybe the s type is not that sophisticated. I will take a photo of my filler cap and post it on the site. Peter.