HI
Have you checked the hoses for any splits
if there's a split in a hose it will cause it to run lean, because its sucking unmetered air from the split and misfire
the easiest way I've found to find splits in induction hoses, is to use easystart, with the engine idling, spray it around the induction hoses, if there's a split hose the easystart causes the revs to raise, giving you a close are to look at
Ideally you want someone with Jaguar IDS/SDD to fault find on it, don't if there's any body on here near you
what engine does your car have
cheers
Joe