OK I'll pull it apart again tomorrow and get my vernier gauge on it and see if I can find something suitable.

I have had another strip down and here's some photos. The pipe was slightly bent and when I straightened it the other day I thought I'd done a lot more damage than it appears now I have inspected it properly. It has a small crack from one of the holes that runs about 90 degrees around the pipe, its not at all loose so I'm wondering if I actually need to repair this as there factory drilled holes in the pipe, two at the bottom & two at the top, would the addition of this small crack make a difference? Or are these holes (and the size of them) critical to the cold start assembly?
imgur.com/a/SXTsEgX    
imgur.com/a/g4YR4Jh

I'd either leave it or try to solder it so crack doesn't grow.

If you can get the bike to start cold, the "choke" system is working good enough.  I wouldn't worry about it.  

Yes I think youre right, I’ll drop a bit of solder on there and leave it at that.