it can be a bit of a bugger to get the the nut off, once the nut is off you can check the key is not broken.
If you have a rattle gun (some of us like myself dont have the liberty of one of these) it will come off very easily. If not I usually soak the nut in wd40 and wrestle with either the blade disc or the flywheel to hold it stopped, being very careful to not break fins on the flywheel.
