yeah mate 19mm, I got under it and fiddled around with it til I managed to get the open-end spanner on it. Spanner was a bit loose, makes me think maybe it's an imperial size? Anyway, I slowly built up pressure and it cracked. Any more and I reckon I would have rounded it but have to take the win. After that I just unscrewed it by hand. The wires had been almost broken by some unknown past encounter so I cut them off and re-soldered them. Worked a treat and didn't cost me anything. I know a new switch is only $30 but no need to drop that coin if the old one still works.