Thanks Extreme,
No signs of any loose fixings or cracks/splits in the pedal; it was knocking against the sleeve of the steering column on the return stroke. It appears that 106K of pedal strokes have bent the entire pedal slightly to the right, so putting my 20 stone to good use, I've bent it back to the left, and now there's no clonk.
Now there's a little 'click'; the hole for the clevis pin and/or the clevis pin itself are worn oval, so I've temprarily adjusted the pedal stop to all but remove the play (still left a little play in it so as not to bear on the hydraulic system permanently.
Anyone got a spare clutch pedal they don't want? Doesn't matter if it's a little worn at the master cylinder clevis - I can weld and redrill if necessary.
Dale