Nice one Sacha what a great little bike thay are.
Well I must agree with DirtydT here while WD40 is a 'must have' great allrounder to have in the garage, a tin of PB Blaster? would be a better penetrating oil because it's what it's made for.
as to the axle, if it comes to striking with a hammer, leave the nut on at first...unscrew it to level with the end of the axle it will prevent you mushrooming the end over and then needing a new one. Also place a stout piece of wood right next to the other end and jam it up to a wall, this will help stop the arm from flexing and more of your efforts will be directed to driving the bolt out