Facelift mk1 C5 Fuel flap not locked message?

[jimmymacD]

Hiya Peeps!
New to me 2008 C5 with a few issues to sort out.
I keep getting the "fuel cap not locked message" and it is slightly annoying, that and the constant rear seat belts locked/unlocked message.
I have taken the filler/flap insert out to check wiring etc.. the only electrical piece I can find is a small square white plastic box attached to the top of the filler neck. I assume from other posts that this is a magnet/sensor which connects with a magnet in the filler cap? There have been various posts about this issue but I can't find the solutions as the links have long gone. Should I get another cap from Citroen or do I need to replace the white piece or can it be bypassed? Any help would be great.

[GiveMeABreak]

The magnetic elements are likely to do with the fuel cap opening sensor used to notify the engine ECU that fuel may have been added when taking into account the fuel level for dosing the eolys additive.

That's separate and nothing to do with the locking mechanism.