HHD I know you are always keen to analyse and get to the bottom of what's going on and I wish more people would do that, rather than relying on rules of thumb and guesswork. OK, in this case I wasn't in agreement that your method was an accurate model but it prompted some discussion and comparing of theory and practice and I think that is often lacking. People do stuff on paper to pass exams, and then nail stuff to the wall, but don't make enough of a connection between them. IMO the real trick is not to get too fixated on regs, which are just the particular locally-recognised standards that we generally work to, but rather to look at the physics that makes electrical stuff work. Heating elements obey ohm's law, not BS7671!
E2A: But, next time, please don't use Q for an unknown current. Capital Q is charge, usually in coulombs. Either x (the general unknown) or I (for current, the actual quantity that was unknown in this case) would suit. Since Q=It, using Q for current makes time disappear, which could cause wider issues with the universe.
I can see what you are getting at, i.e. one cooking station controlled as a unit from a single point, but referring to the isolator specifically seems arbitrary as it has no effect on how the appliances are used.
. Still I enjoyed working it out! 
