Who let the horses out?

[His Dudeness]

I was wondering fuel supply - but dismissed as that would lead to stuttering, which Paul says he isn't getting?

It wouldn't have to stutter at lower speed, it could be that the fuel supply can't keep up with the demand at higher speed/revs. Happened to me once on a long motorway ride, it just ran out of puff above a certain speed and then it started to slow and eventually it ran the carbs dry. If it was a blocked air filter you'd probably see a drop in mpg which you're not so that might point more towards a lack of petrol.