In my 2004 XJ8, it's under the hood, just in front of the passenger-side windshield. This youtube video is for an X-type, but the location and sneaky hiddenness is similar:

 

Holy grave digging Batman!

EDIT: deleted, thought they were asking about fuel filter again

If it's a US car, it probably doesn't have a cabin air filter. That said, it would be in the same area as in the video, just a slightly different location for screws. One of the few things that's probably easier to do with the forward-folding hood instead of the "standard" hood in later cars. Careful not to break the plastic barb for the washer hose at the wiper pivot...