The nut is a normal right hand thread.
(That's the easy bit)
The plastic wiper arm has an aluminium bush,which corrodes onto the spindle.The wiper arm will snap off,leaving the bush firmly stuck in place.
Worst thing is,the spindle comes straight through the window glass,so too much force trying to shift the bush,and your window's gone!
Remove the nut and the old arm.
Then use a junior hacksaw to cut through the bush (it's really soft,and cuts like butter)
Fit the new arm and nut...sorted!
Toploader, July 2010