Thank you all for the replies.
A normally very reliable local supplier measured my pivot bolt and declared it to be M12, but he didn't have them in stock. I ordered minimum quantities (2 bolts and 5 nuts) from Peter Bentley in Paignton trading as "Aruncas" on ebay. Of course they are wrong. With help from the YBW forum I am convinced it is 1/2" UNF. I have just returned the M12 stuff and re-ordered UNF items from the same source. They are A2.
I'll report here when they arrive. If I don't then you'll know I was wrong again and I've probably thrown them through the garage window along with my dummy and all the toys out of my pram!
I could use the old bolt at a push but I want a new one to go with my shiny new laser cut rudder blade. I'll try and update that thread with pics tomorrow.
Floyd