Qualcast 35s - Replacing belt, the nut is stuck

Hello,

I have taken over my fathers Qualcast 35s and discovered I need to replace a belt as it will not roll freely, but is okay when the mower is running.

I have got a correct size belt now, but cannot take the nut off to fit it. I have used WD40, but will not move. The nut in question is circled in yellow on that photo. Any advice would be most helpful.