At the traffic lights at the end of the slip road I half expected it to cut-out, it didn't but the idle seemed a bit lower. I couldn't hear the engine properly as I had headphones in, but it seemed to be struggling slightly at lower revs. Top end pull was fine.
Yesterday I rode the bike in to work, and the engine was chugging and misfiring at low rpm, with the occasional backfire coming on/off the throttle.
I'd had the carbs balanced recently, so checked all the connections under the airbox were secure, checked the TPS setting and all looked good.
Next step check the plugs, and I found this:

Hopefully the bits that are missing exited the engine without causing any damage!
I also need a new front plug cap, as part of it is still attached to the plug.
It's nice when you find the cause of a problem, and it's simple
