how do i fit a new belt to my flymo 330?

I have a wheeled Flymo PC330 mower.
The belt falls off with a few rotations of the blade.
Adjusting the belt makes no difference whatsoever.
Two repair companys have informed me that the plastic chassis on which the motor is mounted is unfit for purpose and it's not fixable.
I've watched a video in the web from Espares & I've followed the easy instructions to the letter.
The problem, it seems to me, is that the main pulley that fits above the blade has no flange to keep the belt on. The vieo & my Flymo 330 lawnmower are the same.
Anyone got any ideas?

Paul
August 2011

Re my above answer:
This relates to a Flymo Vision Compact 330, three years old. Other similar mowers may differ somewhat, i.e. have a different number of screws, and be Torx headed.

Steev
May 2007

Disconnect from mains. Remove collection basket. Remove front orange motor cover, two Philips screws in recesses. Remove blue and brown wires from motor (pull off).
Turn mower upsidedown. Remove blade, spacers & fan, by undoing plastic headed bolt with a spanner.
Undo 8 Philips headed screws. Withdraw motor pulley assembly.
Slacken four Philips headed screws from motor.
Remove old belt. (Optional: check condition of motor brushes, and bearings, also main shaft bearings.)
Fit new belt, and tension (about 5mm deflection when pushed firmly).
Reassemble in reverse order.
Note: Wire colours to motor are not important.

Steev
May 2007