The XJS should have virtually no pressure in the tank. Jaguar had a recall for the earlier cars to correct this in the early 90's. Your car should have already been modified on the assembly line. High pressure causes the tank to flex and crack the seams which causes it to start leaking. Since it is insulated in Jute, all you see at first is a fuel odor but no dripping. Possible causes are an obstructed vent line or faulty one way valve that goes from the tank to the carbon canister. If there has been any curb hopping or someone that misplaced a jack, the line could be crushed. The one way valve which is located near the canister should vent freely in the direction of the tank and have 1-1.5 psi in the other direction.
