[Mothy]

OK - firstly thanks for the feedback regarding my health. Great knowing I have people wishing me well from around the world, not just local friends.

Bit more feedback - I replaced the CDI box today with a factory original (borrowed from a local friend) and the problem did not go away - still just works fine for about 5 mins then as the engine settles into what would normally be full running mode (not warm up mode) it misbehaves.

I have previously changed the coil (I have 3 working coils I can swap)
I have checked the fuel pump circuitry and its all OK - and would not make sense to be cutting in and out once warmed up.

Next item I plan to check is the Supplementary Air Valve - its in parallel with the Warm Up Regulator, so worth a check - I am wondering if just electrically isolating it/disconnecting it will highlight if that is the cause of the problem once everything is warmed up. Otherwise I'll look to borrow one and see if that fixes the issue.