Cleaned and re-gapped the spark plugs tonight. Healthy colour but those gaps were enormous. The book says 1-1.1mm, the gaps were all around 1.7mm. Blasted the air filter with compressed air resulting in all manner of filth coming out. Took it for a hurtle up the road and back, resisting the urge to lay some Os on my mate's gravel truck turn around, and now it runs heaps better right across the rev range.