I have a Flymo Hover Compact 350 mower which is not cutting as well as it
used to: it tends to leave clumps and sometimes I have to go over the same
patch a couple of times because initially it bends the grass over rather
than cutting it. This is for normal lawn grass with maybe a week's growth
since the last time it was cut.

I've had the blade sharpened. As far as I can tell, the motor is running at
full speed.

The only thing that is unusual about the blade is that it is slightly
kinked: the last few centimetres where the blade is sharpened bend upwards
by a few degrees (ie the tip is slightly further away from the grass).
Should the fact that the blade is not perfectly flat matter, given that the
height of the cut will be determined by the lowest point of the sharpened
part of the blade.