Hi everyone. Thought I would bring this thread to a conclusion with an update....
The issue was indeed a single coil that had packed up. I've been in touch with the buyer throughout.
I offered to pay for the parts and labour, but the buyer declined and seemed more than happy to cover it. We've been in comms since and it sounds as though he's enjoying the car and getting plenty more use out of it than me.
For the moment, whilst life is so busy for me, I've picked up an F31 BMW 335D XDrive MSport Touring which has far exceeded expectations and covers everything I "need" from a car.
Hoping by this time next year I will be back in a 911 of some sort.
well, you proved me wrong, your method worked well, and because your buyer knew that you were sincere, support and encouragement was enough for him, well done, palladium.
Cheers.