Thank you Keith. I suspect that the clue to my misunderstanding is indeed the IAS thing for when comparing roll rates at 420KT IAS/TAS at sea level with roll rates at 200KT IAS/420KT TAS at height the sea level roll rate would win out. Hence the statement in the flight manual that for a particular aircraft (albeit a reasonably fast little jet) the optimum rate of roll is at high IAS at sea level.