stick the gearbox studs in a vice with aluminium jaws
give them some heat and VOILA
Realized that I'd cut these studs size so the dome heads would not bottom out so:
Decided to give it a go myself, found and cut some ally and bought a new gas bottle.
Flushed with success I called Alex at Grizzlys who had suggested the method and luckily able to cancel the studs ordered and got some S&S stainless manifold clamps instead.
And a big thanks to Rowan for his kind offer to sort this for me.