I've just fitted a long throttle cable (3.2m) but it binds when I try to pull it. The throttle turns by hand freely and the (disconnected) cable pulls through easily enough. When it's all connected, it just won't pull. Is there a limit to the load and length of these things?

Are you sure its not dragging on something or kinked. Make sure the clevis pin it attaches to can swivel freely as well

I use a 3m long cable without the problem you describe.
Does yours have a plastic liner?

There are no kinks. Thereis a 180 degree turn but it is about 20cm dia. It doesn't have a plastic liner though. Does that make much difference?

As your pedal got a return spring on it or are you relying on the throttle return springs to pull the pedal up ?

I used Teflon lined cables, slippy as hell.