I cannot locate the original post but I have now checked my own XJ6 Series 2 and there are three keys as said earlier. One key fits the driver's and the passenger's front door, one fits the ignition only and the third one fits both the glove box lid and the rear boot lock. I hope this helps.

 

my Series 3 came with 2 sets of 2 keys each.
Ignition key is only for ignition,
Second key works on the doors, trunk, gasoline fillers, and glovebox.

 

One of them, NOT the ignition key, should fit the gas caps.
My car was a Mess when I got it, but it had all three keys.
1 fit ignition and driver door
1 fit trunk/boot, glove box and One gas cap
1 fit passenger door and other gas cap.

I rebuilt the gas cap locks to take the same key with the passenger door.
Glove box and trunk/boot now use the same key.
Ignition and driver door use the same key.
3 keys, now in what seems to me a Logical order.
All good now.

Ignition on left (since replaced with a Proper key), Glove box/trunk in center, gas cap locks on the right.
(';')