What is tripping - is it the MCB for the cooker circuit, or an RCD?
Or is the cooker circuit on an RCBO?
It's just that if you have a slight earth leakage (not uncommon with cookers), this will not trip an MCB, but it will trip an RCBO, or the RCD protecting that circuit.
But in that case, swapping it to a 40A device will not solve anything - you'll still have an earth fault, and the RCD, or a 40A RCBO will still trip.
If it is an MCB which is tripping, then this has nothing to do with a very small earth leakage - to put it in perspective, it takes literally more than a thousand times as much current to trip your MCB than the fault current needed to trip an RCD or RCBO.
So I'm not sure that the electrician's advice makes sense....
What is the rating of the cooker, and what is the size of the cable?
Does it trip when any part of it is turned on, or when all of it is in use?
Have you recently added anything else to the circuit, or is there a socket on the control unit into which you plug another appliance?
Have you recently put up any shelves etc in the vicinity of the cable that supplies the cooker?
Did the electrician test the cooker and the cable separately?