2002 Jaguar XJ8 gas tank cannot fill

After many years we are recently unable to fill the tank - nozzle kicks off like the tank was full. Note that the fill tube comes in low on the tank so unless you are very low there is gas standing in the tube. There is an overflow vent that appears intact. There is another metal tube vent at the top of the tank, but don't know where it goes or what it does. Is there an anti siphon screen in the fill tube that could be partially blocked? If so, how could it be fixed? My independent mechanic says he would have to remove the tank (5 hours labor) to fix the problem. He also says the fill tube is integral to the tank and cannot be separated or repaired independently serviced. Help