This first happened a couple of months ago.
My XJS started fine in the morning, but after stopping briefly at a petrol station the starter would only turn very slowly - not enough to start the car. I turned the key and held it to try and force the starter to turn, but this caused a puff of white smoke to emerge from under the right hand side of the bonnet - exactly where the distributor cap and coil are located.
Shortly after, I tried the ignition again and the starter spun perfectly...but the engine wouldn't fire.
I checked the plugs and found there was neither spark nor fuel getting through.
At that point I gave up and handed the whole problem over to a local dealer who quickly diagnosed the fault as blown fuses to the fuel pump - one under the glovebox, the other in the boot next to the right hand side rear lights. He also replaced/refitted a couple of earth wires.

The starter has jammed a couple of times since, but only briefly and I attributed it to the car having been parked up for several months over winter.

Yesterday exactly the same thing happened again - the slowly turning starter followed by a puff of white smoke, followed by the starter spinning freely, but not firing the engine.
This time I checked all the fuses and none had blown. It seems like there should be a simple solution.....can anyone shed any light on this problem?

Many thanks in advance - Matt