I've contacted 8 online retailers; Rimmers, Canleys, ANG, Robsport, Leacy, James Paddock, Jag Spares and Moss-europe.
I did this just by googling the part number and making my way through.
Most haven't responded, those who did didn't have any stock to measure against, ANG's kit was too short (17 - 18 mm) but the winner is:
James Paddock! Not only did they get back the quickest, but their thread length is 25 - 30mm which is exactly what my mechanic is saying he needs. They are also among the cheapest at £8.95, plus postage of £5.
So they get my order.
Hopefully this might save you all a bit of time when you need to replace your choke cables...