Thanks for your help, but GTT have confirmed that you can't unscrew the torch fitting
'There is no way to remove the stud from the torch and replace with a screw in fitting.
There is no room in the torch for that and also, no fitting with the right size thread. The hose would have to be clamped on to anything coming from the torch.'
And a twelfth plumber has now refused to touch it. So.. anyone want to a buy a brand new Bobcat? Or an oxycon thats still in the box? Beware if you're in the UK and you'd like a setup indoors, unless you know a dodgy/friendly plumber its apparently no longer legal!