You should have a heat deflector which you fix on the top of the oven. It`s just a long piece of metal which you keep in the oven draw when not in use.
With Oven Cleaner perhaps? I find Mr Muscle stuff all indespensible, and think that they have oven cleaner in their range. Failing that you could use vinegar, and see if that works by soaking it on all night.
You should have a heat deflector which you fix on the top of the oven. It`s just a long piece of metal which you keep in the oven draw when not in use.