Surprised to hear that Jonathan.! unusual to develope runout unless seat not cleaned often or not tightened correctly allowing the collet to spin in it's seat. Thou over tightening is just as bad and will distort the nut.
Must say thou I'm very carefull and blow out the nut an clean the taper every time I change a tool, I'm also carefull not to over tighten.

I recently killed a spindle so have a spare nut if you don't find one, I don't really want to sell it but will gladly lend you it when James needs is nut back untill you find one.