Thanks for your response.
I should have made it clearer l am looking at the two speed derailleur only option here so no hub gear attached.
Looking at the spec document for 2 spd derailleur the outer cog should be at 3.1mm and inner cog at 1.8mm.
Does this mean 1/8” outer and 3/32” inner as the listings are in mm not inches?
I have ordered a standard 13 / 16 to see what l am supposed to be working towards then go from there.