Thanks for your prompt reply .
This fan oven has 2 elements only , one for the fan oven the other for the grill., the grill element works fine , the fan oven element is a new one installed today by me ..In-fact it has been a waste of money as the old one had nothing wrong with it and worked exactly the same as this new one.
From cold the fan oven works perfectly , once its hot then switching the oven off ( with its Oven / Grill ? light knob ) and then immediately switching the light on or moving this knob to any position and the Main single house RCB trips ( the other RCBs do not trip), but the main one cuts off all electric. .
So the fan oven element works and heats the oven up .
Hopefully someone will have had the same relatively common problem, and instead junking the oven managed to fix it by finding the cause, there being only so much that can go wrong .