Thanks again, folks.
What little information I have been able to glean about my bike's history is that the previous owner gave it a top-end overhaul as far as the piston rings when he thought decarbonization was necessary, and I imagine he would have examined and dealt with the valve seats at the time - or at least I hope he did! The mileage reading is still under 11,000, and there's plenty of poke in performance terms.
From what you advise, though, it seems there's something else I need to look into the next time the bike is off the road, as I'd never have given the valve seats a thought.