S2 rear heated screen switch( ie the round pull switch)

This has stopped lighting up and hence working. Power to it so fuse ok. How do I get access to the bulb I have tried unscrewing it buts its not happening, I have posted here to avoid breaking it! How should it actually come apart?
Couldn't help myself, mole grips activated, bulb black and out, bulb casing still intact phew!!! I have not touched this before so someone pre 1992 must have got carried away and thought they were torqueing down a cylinder head when they were actually just replacing a bulb