by Morwenna Campbell-Smith The new Flentrop organ at the Royal College of Music in London is still to have its first proper public appearance, as it were, but already it's gathering appreciative comments from those who've had a chance to play it.  I was introduced to the instrument this week by David Graham, Head of Organ at the RCM. The instrument it has replaced in the Amaryllis Fleming Concert Hall was hardly distinguished - a Walker organ originally donated by Parry, with sluggish electric action and  an unsatisfactory detached console.  "It had been enlarged by Harrison & Harrison in the 50s,…