An assembly error caused this problem. The engine had overheated, it was not due to a clearance problem, nor premix ratio, type of oil, air fuel ratio, nor the way it was run in.
I made the mistake of having the air dam within the reed valve assembly 180 degrees out. This meant that the inlet air fuel mixture was shielded from the left hand side of the rear of the piston(inlet side) and subsequently a lack of lubrication caused excessive heat build up and final rooting of piston and cylinder lining.
The white plastic air dam is offset to receive the angled air from the carburettor rubber boot and distibute evenly to the inlet port. If rotated 180 degrees the off set is reversed and shields the left hand side of the engine.
An exspensive lesson learnt, take a bit more time in assembly (it was obvious in hindsight), or get a manual!!
The thing that caught me out ws teh lettering "DX" that I mounted right way up, whereas I should in fact have aligned the two tangs that are provided on the cylinder and mounting plate respectively.
It was through the forums help, discussion with a reconditioner for piston failure diagnosis and the motor bike mechanic for insisting that it was a lack of lubrication or reckless running in that I was able to build a picture of the nature of the failure and locate the now obvious assembly error.