Fan oven not reaching temperature?

Hi,

My Electric Fan Candy single oven is not reaching the required temperature, well actually it does eventually but say after around 2 hours!!! and also I can feel a lot of heat escaping from the top of the door.

I just wondered if anyone knows if its the seal thats missing? (just moved in to new house, so not sure) or the thermstat and roughly how much these cost to have fixed?

Thanks in advance