My throttle cable came supplied with a clamp, which I've fitted, but it only allows the pedal to pull open the throttle. If you lift off quickly, the pedal springs back and over travels away from the clamp but comes back on its own. Is this alright? I could tighten the pedal to give more friction, but that fights the throttle closed spring. Should I get an extra cable clamp to hold the pedal secure from both sides?

i would try to fit a top stop, rather than tightening anything up. Its annoying when it sticks! Maybe put a weaker spring on the carb end? Something must be too strong!