Spot on!not to put the cat amongst the pigeons, having worked in a business that supplied fasteners, there is a reasonable chance that it was a custom order, so you may have to cut a machine screw down to length. I was surprised at the small numbers needed for this kind if order
The 500 piece box arrived today.
It turned out to be M4. Good.
Luckily, I have a Dremel.
Job done.
Just 499 pieces left to use
Thanks for all of your replies.