On the North American Vincent Owners Club NG, there is a thread about non-resistor spark plugs being phased out. Although standard NGK type plugs (B5ES, B6ES or BP5ES, BP6ES) are still available in the UK, you already have to order them by the box of 10.
It might be worth while going out and buying a few of your favourite plugs before they all vanish.
regards,
Kim Siddorn.