I've just had to reset the thermal cutout of my immersion heater. Whilst removing the cover, I've stupidly dropped the nut somewhere behind the tank. Fishing around with a magnet has pulled up all sorts of metal objects, but unfortunately not the nut.
The long threaded screw/bolt in the above picture goes through the hole in the cap. The nut then secures the cap in place.
Will this screw (or threaded bolt, I suppose) normally be a standard size? If I were to try and buy a replacement, what should I be searching for? Or will I have to get hold of some calipers and measure the darn thing?