Is this the right part number for my 1999 XJ8 Base? Sorry I was not able to take a photo.

It looks different from the one in my car when I bought it. Which is this one below.

 

The part number doesn't match the ones called out for in the SNG Barratt catalog. The last letters should be BA or BB. The cap is VIN specific.
Follow this link to their web site and check out the one for your specific VIN.
https://www.sngbarratt.com/us/#!/English/home

 

I think the old type (as per your picture) was a NMB5942AA, which was superseded by NNG5942BB (up to year 2000) and NNG5842AG (for 2001-2003)

The later two types look a little "sleeker" but I'm betting any will fit and work fine.

 

The caps did change a tiny bit .... had to get one last year

 

TSBs for the fuel caps.

bob

 

I replaced mine w/ NNG5942BA which was listed as compatible for XJ6 (91-97) XJ8 (98-03) XK8 (97-00) & has been perfect thus far.

https://www.ebay.com/itm/Jaguar-XJ6-...A/311717659766


I got mine from Welsh on eBay linked above & they've never steered me wrong w/ parts before. Here's the comparison pic between the 2, my new one(on the right) has a bigger footprint.

 

I replaced the gas cap on my 2001 VDP with a NNG5942BB and it has been fine.