Mmm...well, I tried testing the filter unit, luckily I have a spare engine with all the ancilleries to compare
When I tested the spare, using a digital meter, when connecting across the "IGN" terminal in the connection box and earth...I can see about 300ohm, and then increasing within about 1 second to over 1M (all very rough measurements). When I reverse the polarity, I see the same readings. I assume this is the capacitors charging up.
When I checked the one on the ferret, its almost a short (I need to redo those tests with the wires disconnected to make sure I am reading it all correctly), without the ignition switch etc connected.
While I had the coil and connection box opened up, I thought I may as well disconnect the filter unit, and run a wire direct from the "SW" on the coil, to the "IGN" connector in the connection box and give it a run.
Well...its the smoothest its ever run, with none of the spluttering under load. The timing does seem to be out, but will have a look at that tomorrow. I wonder if the capacitors have failed in the filter unit?