It's OUT!
Thanks to the restorative properties of PlugGas (!).
Managed to free it enough to gradually turn the grub screw with what remained of it's head using tiny flat nosed pliers.
Once out, filed the head flat and cut a new slot with a hacksaw.
Cleared the detritus out of the thread in the knob, now it turns smoothly.
All's well that ends well
Mods: If you would like to close the thread now please...