Hi, the description of the repair does not sound correct and the price sounds awfully high. The usual leaks on the XJS are from the steering rack or hoses. The steering rack and hoses are mainly accessed from below the car. I don't see any reason to take apart the motor to access the hoses or the rack. I think what he is describing is the removal of the intake manifold on one side of the motor to access the lines but there is no need for that.
Let me know if you have any questions.