So all finally sorted thanks jaxx!!!
My carb has always been in the same position, ie not fully wedged into the boot.. Had no problems before so kind of dismissed this as the cause. I think this was the issue and maybe wouldn't had needed to change the top end( cylinder and piston).. but maybe for the best that i did.
Here it is fully wedged in. i took the carb and the boot off. fitted the boot to the carb, then bolted the boot in.. a bit laborious but seemed to work
The boot connection had a small split. it doesnt appear to go all the way through but will get a new one anyway
So after I put it all together the things didnt even want to turn over.. even worse than before.. then i Bumped it down my steep drive and nothing.. completely dead.. pushed it up,
again scratched my head, thought it was the spark plug so changed it and it fired up and it got going.. enough for it to spew out all the gunk allowing me to adjust the idle and to keep it running.. whoop whoop