Is it a Transit starter thats faulty or the Mondeo one?
As they use a drop-down gear to transfer power from the motor to flywheel, quite often the grease used to lubricate the shaft dries and seizes the bendix onto the shaft causing motor to turn but not engage onto the flywheel... (starter motor whizzs over)...
To cure it, remove the starter motor, tap out the roll pin in the end of the housing holding the lower gear-shaft in place. Using a pair of mole-grips twist the lower gear-shaft whilst pushing it out of the end plate. Once enough is showing out of the end, use the mole grips to pull it out, whilst holding onto the lower gear.
Now you can see the starter motor bendix, lubricate it initially with WD40 then try moving the gear (bet it's seized), you may need to give a bit of levaridge(sp) with a screwdriver or even a whack!
You can if you wish, remove the end-casing by undoing the solenoid and motor body bolts and remove the body/solenoid as one unit. This is not advisable unless you are confident with refitting as getting the solenoid and rubber solenoid seal in correctly can be a nightmare.