Locking fuel cap replacement Nuevo 2002

Hello everyone, 

I have recently purchased a Nuevo on a 2002 plate, and its not without some issues, some large, some small.
The one I'm finding the most irritating is that the fuel cap does not lock, or even stay on, but it does function with the ignition key, so must have done at some point.

I have searched the web and cannot find anything other than an emergency type(now in place) but this won't pass the next MOT(feeling unsure about the most recent MOT! )

Has anyone bought a generic one that fits properly, its a Boxer base 1.9TD?
Many thanks

Several available  on eBay but 2002 seems to be a changeover year so need to check carefully,  some are screw on others just twist, heres one example which claims it fits all years but check yours is screw on...

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Hello and welcome to this most friendly and informative forum. 

Thanks for the information, I had a sneaking suspicion that it wasn't as straight forward as I thought. 

Now I see the difference in screw fit and not, I can understand why I have had to send two back!
There is a real possible on eBay, really looks exactly the same,  

This might be a fix, if plastic, soak in warm to hot water, it fixed my flask water cap when it would not lock