HA! I am an idiot. I did check the lever movement as suggested by Dutch and while my description as "silky smooth" was technically accurate inasmuch as it seemed to pull and return without binding, I have just applied half a turn of a screwdriver to the pivit screw and POING - the clutch lever returned of its own accord seemingly and now works like a well oiled, er, clutch.
Next time check a suggestion fully rather than thinking, "nope, that feels smooth" and starting to panic about the need for a new push rod, new SRM clutch etc.
Thanks to everyone for all the suggestions - and thank goodness it was the quickest, easiest and cost-free solution.
Dutch - a million thanks and apologies for not actually using a screwdriver straight away as suggested.
Were it not for the 60mph winds and lashing rain I'd be out for a spin straight away, it being my day off, but might be wise to stay in the shed admiring the clutch (and using RGS Bill's simple suggestion for working out whether it is four or six spring).
Many thanks to everyone.