Hi there spc,
Basically, for the RCD to trip it means that current is leaking to earth somewhere that it shouldn't.
It could be faulty wiring, as mentioned already, or it could be an element which is on its last legs. The heating elements sometimes go quite leaky after a few years and cause what appears to be random tripping. Sometimes the insulation within the element can break down so that a short occurs when the element gets hots.
Another possibility is that grease has gotten into a terminal block somewhere under the hob and is causing the tripping -- I've seen that on several occasions.
The electrician who came out should have been able to perform an insulation test on the cooker -- Basically a high-voltage test to see where the leakage is taking place.
As for rewiring so that the cooker is not fed through the RCD, whether this is acceptable or not depends upon the way in which your flat is earthed.
It is allowable with some types of earthing system, so long as certain provisions are met, but if your flat's earth is just to a local ground rod then it will most certainly not be permissible.
