Guys - sorry I forgot to come back and post an update here. My bad, I got the bike going and just never got round to finishing the conversation.
So, in the end, it was two separate issues. I got a 2-stroke genius friend round for the evening and we picked our way through it. Firstly, the carb. My only option was to Helicoil the stripped thread and based on people's advice that I would never get a tight fit I decided to buy another second hand carb. £20 later and it was installed and good to go. The inlet and outlet sizes were slightly smaller so I wrapped tape around the outer diameter to beef them up and make a good seal.
But.. after installing it, we still couldn't get a spark. The carb was good which meant it was the electrics. After a big of head scratching we traced it back to a wire that runs into/out of the ECU unit (I think it's called an ECU?). The wire had been resting against the fan and over time the fan blades had been chopping it up until it was only being held on by one copper wire which snapped the second we found it. Soldering kit out, heat shrink wrap connecter thingies and we were back in business.
Bike has been running great since - thanks for everyone's input here on the forum. A few of us have got trials bikes at the same time up here and have been out practicing. The plan is to get our ACU licences in the new year and start entering trials. Till then it's pay-n-play and Fuddy Muckers up near Conset. And to prove the bike is going again, a little pic I snapped of my mate Joe on Sunday. I did however manage to drop it and crack the clutch master cylinder. It will hold with a bodge repair of metal glue and zip ties but I'd rather put a new one on. Don't suppose anyone has one to sell before I order a new one?
: )