Thanks for the tip from this forum. New plug caps sorted my bike too
I bought it at 2 years old and 520 miles - wouldn't have thought there would be such a problem on such low mileage - but hey, if the original caps were duff, what can you do. You could definitely hear the "clacking" sound of the spark escaping before they were changed.
Care on the rear cylinder, though - the NGK caps I got were quite long and the routing for the plug cable is close to the throttle mechanism - you don't want those two getting together!