Well it's taken me a while, but last week I finally bought a K&N filter, Hitchcocks airbox cover and a PCV. After fitting these (still using the RE sports silencer) the bike definitely performs a bit better, particularly after 4000 rpm. However, after 2-3 miles the tick-over rises to about 1800 rpm and doesn't return to normal until the bike is next started from cold. Any thoughts/suggestions? The spark plug looks a nice healthy colour.