Any question answered...

Today I noticed a sort of roughness in my drivetrain. I've cleaned, tightened and oiled (as appropriate) the pedals, crank bolts, chainring, cog and chain. No luck. It only seems to happen when I apply proper force through the drivetrain - just letting my legs spin with the cranks it doesn't really happen.

Never noticed it before - it came on quite suddenly.

I've not stripped it down to the BB yet. Reckon it could be grit or looseness in the BB somewhere?

Is the rear wheel definitely straight? If you've pulled it slightly off-centre it will have adjusted the angle of the cog against the chain slightly.

Sounds too obvious to be the case though, and I guess if you've been fertling with the chain that's probably not it.