Would incorrect valve clearances cause problems with idling, i.e. high idling? My simple logic would suggest this perhaps

Dont think so. It normally just causes a tapping noise. Have you checked your idle control valve? If not, remove, turn ignition on and the valve should fully retract and then settle back into position.

how incorect?

if adjusted so the valves never close, you will get poor compression so poor running.

If the valve clearances are too tight you will have problems getting the engine to idle , high fuel consumption and lack of power .Not a too fast idle. Over advanced ignition timing or a badly adjusted carb/throttle body would be the first things to check although correct valve clearances are important