How do I remove the complete ignition assembly?

Ford Transit Van, 1996
I need to remove the completely aluminium ignition assemble from the steering column as the tibbi ignition tumbler switch is completely worn and take it to the lock smith. The complete assemble seems to slide over the steering shaft and there is a lock circular clip and spring compressed that I think is holding it in place, but not sure if is is part of the securing technique. There is no external screws etc holding the assembly onto the shaft. It slides over the steering shaft and enters within the aluminium assembly. There seems to be no obvious pin hole either for removing the key barrel.
I am wondering also if the key needs to be inserted to release, but the tumblers inside are in bad way as the key may or may not be easily removed as it jams and the starting procedure is erratic...could not turn the ignition off before and had to disconnect the battery.
Any prompt assist would be hugely appreciated