Summat is bent or broken off.
Is the blade bent/broken?
Checnk the fan above the blede: is that broken?
Take the blade off, put the nut back and run it like that for a mo. If it still vibrates, it wasn't the blade.
Unplug, turn the mower over and rotate the shaft by hand. Does the shaft look like it is spinning with its end off-centre?
This place has some diagrams which may give a cluse as to how the thing goes together:
http://www.gardenhirespares.co.uk/c/shop_by_parts_diagram/flymo_mower_parts
XXXXX
September 2015
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