If you haven’t tried using baking soda (sodium bicarbonate) in cleaning, it’s well worth a crack. I scoured the oven baking tray last weekend which was covered in old spots where oil had been repeatedly baked on to the surface. I first removed what surface oil I could using boiling water and washing up liquid, but when all that was removed, there was still a patina of older deposits. Baking soda mixed into a paste with a little water and dabbed onto the desperate parts of the tray and then scoured off with a little elbow grease. You would not believe the difference it made. The tray looks brand new again. I wish I’d taken before and after photos.