Thanks Phil, all major connections so far look good, and certainly the ignition switched need a clean, but I would not think it would start immediatly again if there was small issue such as a loose wire to the distributor, but I've been there in the past so I need to pull the bulk head cover and have a look in there.
As I go through these issues, I'm finding the car is pretty sorted so quite unlikely for there to be an obvious loose wire, whereas reading around the Holley carb setup and a lean idle it seems can cause issues particularly with a performance cam which my car has.