Why would the engine on a 1993 Jaguar XJ6 turn over but not start?

I HAD THE SAME PROBLEM AFTER I RAN OUT OF FUEL AND I WAS INSTRUCTED TO LEAVE MY IGNITION ON FOR AROUND FIVE MINUTES TO PRIME THE FUEL AND TO MY GREAT PLEASURE IT DID THE TRICK, I DON'T KNOW IF THIS IS THE SAME PROBLEM YOU HAVE BUT GIVE IT A SHOT ,HOPE THIS HAS BEEN HELPFUL FOR YOU.GOOD LUCK.

My XJ8 refused to start after switching it off after only running it for a few seconds; I was told the cylinders were flooded with fuel (when the engine is cold more fuel is pumped into the cylinders).

The remedy was to start the engine with the gas pedal flat on the floor (don't 'pump' it); this will stop any further fuel being sent to the cylinders and will also clear them of any excess fuel. When the engine starts just take your foot off the pedal and let it warm up. Hope this helps.