As this is related to my general issue, i thought i'd post the Update here too in case of use to any others in due course:
stripping back the carb off the manifold reveals the previous owner or the garage (known to me) shows that again they are allergic to gaskets:
Not only the exhaust gasket missing (found out that a while back) but also
no gasket between the bakelite heat shield and manifold
no gasket between the carb base and the bakelite shield
carb retaining nuts finger tight.
yet there was no pick up when spraying engine start around the joints....
I sincerely hope this is the cause of the running for 20 mins then cut out, cool down for 30 minutes, run for 20 mins, repeat....?
The primary main jet has been increased from 107 to 110 but i will post separately re that.
cheers
RDS