How to fix Belling Oven-Model Synergie Mulitfunction?

For some reason the oven is not heating up correctly. IE even on max, temp. it took 3 hours to cook a dinner than would normally take only 1 hour.This is on both normal and fan settings.
On wednesday the house electrical sockets all fused and the oven was on so I wonder if this could be related. Especially as the next time I used the oven I noticed the problem.