My smeg uk80mfx1 trips the rcd after 20 mins or so?

Hi
Smeg oven uk80mfx1
On the fan oven setting the cooker cuts out and trips the house rcd after 20 mins or so The oven has heated up by this point and the fan is spinning.
This does not happen on the non fan setting with top and bottom elements.
Any ideas what is causing this and how to fix?
I have replaced the heater element for this cooker before but unlike this situation when it goes - it's gone and does not heat at all.

Andy A
July 2013