Thanks for the replies folks.

"try new IIRC at 3600 rpm it doesn't take much out of balance to rattle"

can you please explain what the above means (IIRC)?

I have tried to watch the blade retaining nut whilst trying to pull the cord but it's really difficult. I also turned the blade by hand but couldn't detect any non-concentric movement.

Would running the mower without a blade attached show if it's the blade causing the problems?

I have noticed it makes quite a loud 'clunking-type rattle' just as the engine stops - any more thoughts please?