Just as I thought I was getting to the bottom of her cold, things progressed to Man-flu!!!
Completely cut out in the fast lane of the M4. 4.5K revs to nowt in a split second. Dropped a couple of gears and bumped her but she took an age to fire again and even then she was coughing and spluttering for quite a while.
A minute later all was well, not a hitch, smooth as you like and sounding sweet! And so it remained for the rest of the journey

When I got home I gave GC a bell for some clues and he suggested a nosey around the coil where I did discover a split in the insulation of the live feed in (worn though). I've vasellined it and covered it with some heat shrink as well as vasellining all connections and checking the plug (which seemed fine)
Here's hoping I've found the cure. Do you reckon that could be it?? Anything else to look at??
BTW GC I took a look at the filler cap breather and all was clear