Success!! I took all terminals off all elements. Turned on the power, no trip of RCD therefore switch okay. I then connected just one of the ring elements round one fan, again turned on and no trip, it warmed up. I then added the other ring element, but it tripped. I removed that element and connected the next, grill element, tripped again, then finally the bottom element, trip again. I decided to leave the one ring element on and put the oven on full power and left for half an hour. Then I attached the other ring elemet again, turned on and no trip this time! Then after another 15mins on full temp I attached the grill and bottom elements, hey presto no trip of RCD. Oven all back together and working fine, cooked dinner and everything fine and dandy.
I phoned Service 71 in Haywards Heath to tell them I did not need their engineer and to say how annoyed I was that I had now paid £40+ for a switch when it was a most basic of problems. No comment from Service 71 but they can think again if they think I will pay any bill for their visit when they could have checked the elements properly and saved me a lot of hassle.
If anyone wants a switch for this cooker let me know. I ca't see them going wrong very often as they are very simple engineering.
Thanks to all who replied to my query. Considering Service 71 is the only agent authorised to service SMEG in my area I am not impressed.