If they look like the first photo below, part # MWC9919, Buffer Fuel Filler Door, Rovers North lists them for $2.27, but have found them elsewhere for over $12 apiece...

http://www.roversnorth.com/ProductDesc.aspx?code=MWC9919&type=0&eq=&desc=BUFFER-FUEL-FILLER-DOOR

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If looking like the pic below, Rimmer Bros. (UK) lists them as Fuel Filler Flap Rubber Buffer, part # 614557 though a quick search leads me to believe that may be a "house" part number...

http://www.rimmerbros.co.uk/Item--i-614557

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Mine actually look like neither. They appear to be just a slip fit bumper cover

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Compare it to your bonnet buffers (the ones covering bolt-heads at the lower-side bonnet corners).

 

LOL, already did that! Bonnet buffers are noticeably larger in diameter.