I've tested the plug leads and two of them show resistances of 5k or less, but two of them are well over 10k; with one of that pair nearly 16k Ohms. No idea what has caused the recent ignition issues, but clearly the leads are no longer optimal. I do have some of the original HT leads that came on the distributor I got with the engine - I can probably make them fit the coilpack temporarily to see if that improves things or at least moves the issue from one cylinder to another.