[Gwiz]

Thanks to all who offered advice.  Last night decided to bite the bullet :- drained the fuel tank, stripped the carb and checked jet/needle , diaphragm, all fuel ways and strainers but couldn't see any blockages or dirt.  Took out the fuel tank filter and poked out all fuel and breather pipes.  finally re-assembled put in fresh fuel, set up carb adjustment screws as per instruction manual and  checked operation of throttle lever.
The thing fired up straight away and after warm up revved its little heart out.  It's annoying not to find the definite problem but the little beast's running fine now.  It could  have been iffy fuel or carb adjustment or a combination of both. I did ponder on Nosheds suggestion to bin it and
go back to the spade but its nice and red and shiny and makes nice noises when I pull the stringy thing on top so I think I'll keep it.
Regards to all
Yodapete

I was just about to suggest that.