To perform any maintenance on the ignition system other than cleaning or adjusting the points, it is necessary to remove the flywheel. This is best achieved by jamming the flywheel (to prevent its rotation) with a wooden wedge between its outer circumference and a mower cross bar.
Then, using an appropriate ring spanner, give a sharp strike to the ring spanner with a hammer in an anticlockwise direction to shock the nut undone (see image). A 1/2in Whitworth ring spanner can be used however, if one can be found, Villiers also produced a special spanner (called a Hammertite spanner) for this specific task. This has a ring spanner at one and a place to hit with a hammer at the other.
If successful, the nut will undo by about one or two full turns before apparently locking-up again. You need to shock this anticlockwise again which will then prise the conical flywheel mount from the mating taper on the crankshaft; the nut is captive but rotates within the flywheel.
The magneto flywheel contains a pair of permanent magnets and the original Villiers maintenance advice was to place a piece of iron (e.g. a spanner) to bridge across the poles thus preventing loss of magnetic flux.