When I did my 16th course the IEE guidance notes for tutors specified it was per appliance, and the example was for a circuit containing an oven and a hob.No, you've not misunderstood anything, but I had completely missed and/or forgotten it !
People are obviously free to apply diversity separately to each appliance, and then add up the after-diversity currents, if they wish to - but such an approach will be much less 'generous' (in terms of diversity) than if one did it for the total cooking appliance load on the circuit - since if one does it separately for each appliance,one's after-diversity figures includes a "10A minimum" for each and every appliance (whereas there is only one "10A" if all appliances are considered together).
As an example,with Sunrays figures above, if both loads are on the same circuit, then applying diversity to the two together would result in an after-diversity figure of about 21.5A, not much more than half Sunray's 40.5A figures when the appliances are considered separately.
Kind Regards, John
The difference should have come out exactly 20A different