Just before the weekend I discovered, quite by chance, that I've got this problem. Fortunately for me it's stuck in the unlocked position.
I discovered it by idly pulling the flap corner while car was locked, it opened! That means any scrote could come along in the middle of the night with a bit of tubing and help themselves to my petrol.
Either the plunger has stuck that way just by chance (?), or someone has forced it up so can then get access to filler cap, and sort it out more properly later (or, I'm trading the car in anyway so s*d it).
Now see this:
This guy reckons forget about making the flap lock work properly, it's all plastic anyway (personally I can't see it resisting much being levered open by a big screwdriver), just disable the solenoid gizmo and instead change the actual filler cap inside for a locking one.
Well I ordered one on Friday, but as I'm not allowed to advertise on here, you could try Googling "Renault Clio Mar 1998 Onwards Locking Fuel Cap PLC89_1"
Or, PM me if you want to know who I ordered it from ~ using their make/model/year search page(s), I selected it as Clio 3, 2010 (year).
Postman brought it today and it fits perfectly! Comes with two keys and cost about £18. You could probably find it on
as well (the outer cap is black, the inner rotating part with the latching tongues is red). The key cannot be removed in the unlocked position and you also need it to actually turn it to latch so you can't make any mistakes.
Proper job, I iz well chuffed with it.