[Defender 110 TD5 Fuel Filler Cap (larger one!)]

Hi, Microcat says I need part WLD100730. I've tried the WLD500200 cap which supposedly superceeds that, and it comes up too small, it doesn't lock properly and fumes escape.

It's a defender 110 TD5, SALLDHA87XA176***

So, what cap dya think I need, that comes up a bit bigger? Maybe land rover found a few old fuel filler necks and blew the dust off to fit one to my 99!

Also, what is with vented/unvented, how dya tell which you need? And it needs to be locking