It does sound like you've got a worn valve seat, or a worn seating face on the valve itself.
If you are sure that you have less than the minimum clearance, then you will need to remove the head. Its possible that the valve has "mushroomed" and the seat itself is OK, or that the seat has worn.
Either way, I would personally have all the seats recut by a specialist, and fit new valves.
Before you spend money on the head, first check that the valve guides and cam bearings are all OK. Also check that you can't get a good used head for less money, for instance from Triumph-ant Motorcycles:
Link
I would NOT advise that you do a single valve, as you could then end up doing the same job again in 6 months!
Good Luck.
