Go on line to something like eSpares and buy a trigger pack of professional oven cleaner for about £7 inc P&P strip the oven out and use it on everythign that is oven cleaner safe. carefully using cloves leave it for several hours and hot soapy water was hit off , your oven will be like new .
From then on use a cassserole /lidded pot for roasting or roasting bags .
If you heat PIZZA in the oven slip a sheet of baking parchment under the slab so it does not burn onto the shelf wires.
If you open pan roast a veg medly slip that parchment on or use a roasting bag.
once you have cooked int eh oven slip a dish of hot water in it and leave it to cool down a bit then wipe out your oven with a slightly damp soapy cloth and anothe wipoe with it rinsed out.
The secret of keeping your oven clean is to clean it everytime you use it , prevent splashed of fat or food and never cook in a dirty oven as it bakes things on.
That professional cleaner from e Spares I spoke of is really the dogs . I've never found anything better .
The label shows :-
" Professional oven care 500 ml "
approved by Indisit , Hotpoint and Scholties ...
NOTE WELL
You must wear decent rubber gloves when using it and wash offf any splashes or smears as it stings like mad on bare skin