Just sorted it, I think
Ran the engine for full for 1 minute with lots of pressure in the recoil still, happy days !
Basically the problem is, as the starter wears a little, in the recoil spring area , the whole assembly is too tight up together.
As the engine reaches a natural frequency the recoil spring vibrates against the metal back plate of the starter, thus uncoiling the spring, its a very strange one but thats the problem it seems.
I have put a small fibre washer between the backplate and the recoil spring and then 2 lock washers between the bolt and the backplate, pics attached, the whole assembly is now spaced away from the back plate so the spring can not touch, I would think machining some washers to fit would more suitable but its all I had at the time and I want to go flying now !
This works for me, hope it works for someone else, anyone using a Flash started should perhaps check this and think about making the mod before your starter ends up in the prop, seems to me a bit of a design fault.... !