Thank You Deejay, I checked the weld and it does seem to be right through. The Allen pivot screw unscrews quite easily, and the hole in which it fits is now a freshly drilled hole in what appears to be solid aluminium.
I took the cutting unit to the sharpeners, and they confirmed that they can replace the bottom blade as you said. These are the people who sharpen Toro greens mowers and fairway mowers, and whose machine I mentioned above. The machine sharpens each blade separately, running along it lengthwise. It must be very accurate, as it sharpens only half the thickness of the blade in order to conserve metal. They sharpened the three gangs of my Toro mower (just backlapping), and those gangs cut newspaper beautifully now.
I will see if I can make a video of the process when my Scott Bonnar is being done.
Cheers MacCullum
