I can't undo the main nut on the spindle, the whole shaft seems to turn. Can anyone suggest how to remove the motor.

I had this problem a while back when I changed over the facia board.

I think I used WD40.
Is it a reverse thread on the bolt?

I don't think it is a left-hand thread - just a nut that unscrews in the normal way. I presume you have the wiper arm off already. The large outside nut should really not be all that tight. There are two Philips screws to remove - unfortunately, those often seize in place, so use a screwdriver that fits really well. Maybe a large screwdriver bit and a spanner to give best leverage.

-Alex

The two screws that go through the cover really do sieze in place - we had to use an impact hammer on one

whoops... I meant an impact screwdriver, but it's too late to go back and edit