I deleted my previous post because it was garbled nonsense.

I started to lose my marbles because I couldn't determine why the stop wire had a fluctuating resistance value. I closely examined the last section of the wire, and the insulation had a tiny cut grounding to earth!  It moved slightly when I measured the wire, which caused the fluctuating readings 

The Mountfield rides on! 

I struggled to get it running because it was a mixture of many issues. The coil had a single filament of wire which had rusted onto the kill switch spade and shorted against the engine. The kill switch wire had a tiny cut which kept earthing. The fuel had water contamination, and the coil was so fussy I had to keep adjusting the air gap to achieve optimum spark.

On top of that, the control module had a dry joint 

It now starts on the first turn, and the client is delighted.

That's a good result then Changy, well done on sticking with it and not giving up.