Hi Stork, thanks for the explanation.
I am as sure as I can be that there is no air leak either from the old oil inlet or around the air box carb junction or the engine side. The bike starts as it always has, first or second kick, no problems. It runs fine up to quarter throttle then starts to run rough. It produces a lot of exhaust at all throttle openings.
As per your e-mail I checked the air filter, the ignition is on the same mark as it has been and I used a fresh batch of 2T mixed as accurately as I could (50:1 synthetic Castrol). The needle clip is set on the lowest point on the needle so I can't lift it further.
I will try a plug chop tomorrow.
Cheers
Iain