I recently got a Jaguar 89 XJ6, and found it leaking gas from the charcoal canister. My guess is that the fuel being drawn from one tank is being returned to the wrong tank, resulting in it being overfilled. I have a shop manual on order, but not received. Any idea where the return line solinoids are, how to check them. Could this be a problem at the tank selector switch? Not real happy about driving with raw gas leaking.
Apparently your canister purge control system is not functioning. Trace the purge line from the canister to the intake manifold looking for a collapse or break.
Also, check vacuum thermal valve(s) on the thermostat housing being they are opening when engine is warm. If the venting system is not working, the fuel vapor from the tank will condense into liquid in the canister.