Thanks Peter, a 60 deg Metric thread pitch fits too but I assumed being vintage Britishicon that it was more likely Whitworth. And you know what they say about assumption.... I also need to learn how to cut threads on the lathe rather than using a die on the lathe and turning the chuck by hand (I have been watching Utube vids to get the idea)

Thanks for the reply too Patrick, I think 1.562" is 1 and 9/16 though. I'm assuming (again) that the diameter of the jacket originates from the US Colt ANM2 .30 that the .303 was based on, I can't be sure though as I don't know anyone who has one.

I can see me taking my part finished replacement to a more competent engineer than myself. At least I can take it and not sound a complete idiot when I ask them to cut a thread on it.

Thanks, Chaps.