This is the cable that I spent half of the morning looking for.
It's old and a bit tatty but it works fine, and I knew that I had it somewhere. I have used it since we moved house so I knew it was here. I looked where it usually lived. I looked where it might be put if it had been used recently. I looked in all the places that it could be put accidentally. With USB cables. With flash guns. With old film cameras that it could not possibly be used with.
Such a simple cable. Three wires, a plug on one end, and a two position switch on the other. I could probably make one with the bits I have lying around, but it should be there. I know it should.
@stanleythehippo Nah - if you only knew how this little cable messed with my head yesterday, and the amount of my life I wasted on it then you would understand that this is probably the most appropriate photo that I have posted of the 17 photos to date.