Oven tripping out electrical circuit when turned on.

I would agree that the most likely faullt is live or neutral to earth leakage on one of the elements. This is normally caused by a microscopic holes developing in the outer surface of the element allowing moisture to get in, and thus reduce its electrical resistance to earth. Only real solution is to test the suspecet faulty element and if its found to have a low resistance either L or N to earth, then replacing it with a new one. This sort of fault is quite common on cookers that are a few years old.

Most likely the element or fan (if fan assisted)

The only other thing i could think of is neutral/earth fault on the cooker cable itself and it may only be tripping the RCD when some load is applied ie oven being switched on.