Thought it was.

So the bikes already got a BMC air filter, and it's in good condition, so that's not the culprit. The bikes only got 8400km on the clock, but a fair amount of that is track kilometers, so the clearances could be fairly tight, but it's a painful job to check them, so I'll leave them.

I might give the battery a go. Where does someone go to get a relatively cheap lightweight lithium battery?