Awaken

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