It now appears that after some five or six years this thread may now be put to bed.
There were originally two engines involved and, after finding one had a cracked piston on which the bottom part of the skirt broke away, it was taken by sunshine man for some parts to complete his unique one off aircooled version.
Following long periods of hibernation and, having taken on board some valuable advice from sunshine man, together with my own general learning by the trial and error method, the engine has now changed from an absolute mongrel destined for the tip, to one that will hopefully soon be a little sweetie. It now starts instantly and runs quite well. With some finer adjustments to the air valve and carburettor it will probably only get better.
Thanks to all respondents for your various comments on this thread over the years.