I'll try your suggestions, much appreciated, I should have at least thought of Teo Lamers what a dullard I am.
Pete No, it's the common or garden stake nut that pulls up the layshaft and holds the worm drive & gearbox endcase in place
That's why I was so surprised that they were even remotely difficult to source, you need that nut if you crack the stake and that's gotta be reasonably common (or is it just me that has fists of ham and fingers of sausage?)
I had to remove the worm drive to get out a speedo drive thrust washer, the dreaded bottom one, actually I found 2 but that's by the by.
What a half assed piece of design that is, even knowing about perils of the washer dropping and so being super careful, I still managed to drop it into the worm drive