Since my last post the bike has covered 1,200 miles - as I took part in the inaugural Vincent Riders Dinner at Tumbarumba, NSW. With the new gaskets installed and the "O" rings in place, along with felt washers there was not even a trace of a leak from the gearbox - so the effort was well worth it. Also with use, the gear-change mech freed up considerably and now feels as it did prior to the "O" rings being installed.
The gear box modification (if you could call it that) will feature in a future edition of 998.
Martyn