1990 Jaguar XJ6 key will not unlock the door

From the way you describe it, you have a key tumbler problem inside the lock. Meaning, with the key inserted the tumblers move normally to lock the door but get hung up on a "bur" or obstruction going the other way. If you pull the handle lock assm. Out (it's a lot of work), you should be able to dis-assemble the lock unit and see what the problem is. Or, you could change the unit but, you will wind up with another key thats just for that door.