Update to close the thread. I removed the engine and stripped the valve gear to take a look at the studs. I located the copper sealing washers which weren't in the correct place (my manual made no mention of sealing washers and cap nuts). I removed the Kibblewhite nuts on studs 5-8, annealed the sealing washers and put them on studs 5-8 along with the correct cap nuts, and reassembled the bike.
Problem solved. The engine is now leak-free. It is rather surprising just how much oil can find its way down a stud thread and under a standard steel washer.
I'm now off to burn my 'Honda' manual and the Haynes version, and locate a real one. Many thanks for all your replies.