Andy,
as I understand it the, temperature rating of the cables applies to
the whole cable being at the higher temperature. When a small local ambient
temperature variation occurs, the derating factors do not, IMO, apply,
provided the temperature at which the insulation is breached does not occur.
The best example I can think of, is the domestic iron, where there is a
major temperature gradient between the sole plate and the cable cores. I do
agree however that at the back of a built in oven, care in cable type is
required. I believe that in the majority of built in oven installations, the
back is actually fan cooled, to alleviate this problem. Anyway, that's why
there is a fuse??!!
The cable size will make zero difference in the majority of
installations, as the I2R losses are too low to be significant against heat
loss from the oven.

Regards
Capitol