Half way thru my ride this afternoon, my chain starts jumping around in the smallest rear cog in both middle and big front rings. It started a few minutes after getting a whole load of bracken snagged in there. Gave it an "on-trail" clean but couldn't solve the skipping. Not sure if the teeth are worn (cassette about 9-12 months old) or if there is something else I should be checking.
Appreciate this is a v basic question but I am a mechanical numpty. Any ideas that I should check before visiting LBS?