[Laird Scooby]

Originally Posted by Clifford Pope

Update:

Having replaced the plugs, tried swapping in a complete ignition set with cap from the last time I needlessly replaced them, cleaned again unnecessarily fuses 4 and 6, needlessly replaced the fuel pump relay, run injector cleaner through the system, none of which made any difference, I think I finally may have cracked it.

I noticed the radio suppression capacitor on the coil, and tried unplugging it, for want of anything else to do, and the the result was an instant and dramatic improvement to the idle smoothness.
I've never read anything about them, just grown accustomed to odd components being earthed with capacitors. No one else has ever reported any faults, so checking them has never featured on any to do list.

Which coil terminal was it connected to?