Many thanks for your replies all
So i had LOTS of fun trying to sort through this today, culminating in a breakdown (due to another electrical issue!)
I have checked the HT leads, and they are all within spec (according to the values in Haynes). I did find number1 HT was quite corroded (the contact, see picture. Good one on the left) compared to the others, so i have given that a clean but will likely replace soon. I've swapped the coil around, and i THINK the issue followed the 'bad coil'... it's hard to tell, but more riding will tell.
The 'breakdown' was the bike just cutting out, and going basically down to one cylinder... to keep it going, i basically had to pin the throttle to even make 50mph... i was not happy! As soon as i was letting the revs drop off, the bike would almost stall, and then wouldn't restart.
So reading elsewhere on the forum, i've narrowed it to either the HT coils (new set ordered!) or the pickup coil. I've since had the cover off the engine, and have bang on spec resistance across the pickup coil (560ohms), but the spacing was a little tight, so i've reset that to .06mm (not that that would have caused the issue)
I ALSO found i hadn't tightened the clamps around the num1 carb rubber (between engine and carb) so they were loose and leaking... found that whilst checking the airbox drain, crank breather and tank breather pipe were all free. which they are.
So, plenty worked through with your help thanks... Will be very interesting to see how she performs and if the issue's got any better, before swapping out the HT coil. The only other thing, to add into the mix is that i have swapped the old instruments out for a smaller digital speedo (a motogadget) so there currently isn't any load on the HT coil 1, where the tacho would have been coming off of. Don't support that would make a difference, would it?