[FatBoy]

Thanks guys for all your input.

The exhausts are fine i.e. not plugged.
The ignition is an MSD 6AL with an MSD billet distributor.
It starts up and ticks over just fine after a few seconds of warming up.
I'm inclined to think it is fuel starvation. The plugs are very clean (possibly too clean). I'm going to pull the carb today to make sure there isn't any dirt in it.
With regard to "reluctant to rev", by that I mean it lacks power and starts to miss above 3,000rpm.

Thanks again.
Paul