Jack,
No I didn't change the wheels, as this would have led to other necessary changes such as the valve gear being altered. In fact altering the wheels leads to a Hornets nest of further problems which would end up requiring a totally new chassis, which would then render the bodyshell useless. The reason is that the driving wheel axle spacing is the same on the S15 & H15 but not on the King Arthur which had a 7 inch longer chassis. Even using the Hornby S15 with a wheel change would not have achieved that.
The options, were basically: A DJH kit and get it exactly correct visually, but suffer the problems of no motor powerful enough that will fit inside the model to haul 40 wagons up a 1 in 100 gradient.
Or: Do what I have done and hack a Hornby King Arthur to produce a model that is convincing enough to pass as an H15, and get a decent powerful mechanism that I know can haul the necessary loads.