the fault is unlikely to be the actual oven unit so therefore in order of least expense the things to check are:- the door seal being complete, the hinges not having been forced, the door not being buckled, self adhesive universal seals are available online from *spares etc. Hinges are easy to remove from the cooker and then can be unscrewed from the door and replaced. if it's the door I would have to say that's less likely to be available for a sensible price.
