Britbrat, have you ruled out crank endfloat as a possible cause of noise at idle?

If not, try loading the crank at idle by pushing the spinner toward the engine (all standard safety waivers apply).

I'd also try a few different glowplugs (OS Type F, OS #8, K&B 1L and McCoy MC-59 should all work well in that engine) to see if changing ignition parameters influences the knocking characteristic.

Good luck.