Quote: "The trip occurs when just the fan and internal oven light come on but before any heaters are switched on "
I suspect that if you megger the oven element you will find it leaky.
They are mineral insulated and not particularly well sealed at the ends. Moisture is absorbed during periods not in use. Hence as soon as another load on the same circuit raises the (un-switched) neutral potential a little, out goes the RCD.
If you once get the element to heat up the moisture will be dispersed further through the insulation and/or driven out, and all will seem well - for a while.
I once experimented with such an element, heating it up then connecting it to a megger. The insulation initially showed infinity, but dropped like a stone as the element cooled to well under 1Mohm.