i am answering my own question, i put the machine on its side, lubricated at the nut and behind the blade, 'carefully' put your foot against the blade to stop it turning, the bolt loosens anti-clockwise, leave the oil to penetrate for a while, the nut (well bolt really) is quite soft, take care not to round off the square edges, i'm taking mine to a blacksmith (it is blunt and bent), if not a new blade is around £15 (uk)
chris, July 2011
