This will smell a bit but is fabulous at cleaning the likes of BBQ racks with no effort. Get some ammonia from, say, Boots. Put some on a cloth, then put the cloth in the oven and leave overnight. The next day the burned on stuff should just flake off - obviously have all doors/windows open and, if you can get one, wear a mask while you can't take the oven outside! With the likes of BBQ racks, put the soaked cloth and rack in a bin-bag, leave outside, clean outside etc.