not a clue what i'm doing with the 2 speed clutch, so haven't dared touch it.

I just re-installed my one speed and that works fine, so the problem definately lies with the 2 speed transmission in some way. It's almost as if the highest speed gear is permanently enagaed, because the motor just can't get any revs going and the gearing seems rubbish because the acceleration is lame.

I bought it from t'internet along with some other little bits (like tuned metal exhaust and manifold - a godsend because it's stopped the leaking the standard one had!).