In article ,
Mr Fuxit wrote:

You've got the blade upside-down! The blade ends should point DOWN.

That was my first guess as well. Blades usually have an inscription
which indicates (or says) "This side to mower". It's often obscured by
years of mulched grass, so scrape the blade to find out.

As for a spanner: surely someone that uses this news group has a spanner
(or rather adjustable wrench) that will undo the nut? You need to block
the blade from turning, if you need to undo it: use a piece o'wood.
HTH
John