If you’ve ever run an old Jaguar XJ40 and had time to study the slow decomposition of its bootlid while waiting for the breakdown man to unravel its electrics, you’ll probably think that the idea of adding a supercharger to this mechanical mix is about as sensible as asking an octogenarian to take up the sport of stair-running. There’s likely to be a lot of wheezing and gasping, followed by moments both cathartic and potentially tragic.