TOPIC:

I have not worked on those specific carbs, but I have not seen any other carbs where the needle jet just falls out when the carb is inverted. That may not contribute to starting problems, but I think it deserves some attention.

Nessism: When you removed the pilot jets and bleed pipes did you use a wire through them to make sure the orifice is open?

No, I didn't. I had assumed that soaking the jets in carb cleaner and blasting them with compressed air would do the job...Lesson learned, and have done the job proper now and unsurprisingly, the one that was working had daylight showing through the jets, but the other didn't. All is good now with the needle jet now staying in position when the vacuum slider goes up.
I have ordered a rebuild kit so will do it all again when that arrives, but for now, where there wasn't any firing without the use of engine start, there is now. Still heaps of work to do, but this is a great break-through.
Thanks all for your comments, super appreciated