Go onto the Lakeland website. Oven Mate oven cleaner and Oven Mate Just For Racks. They’re both probably the best cleaners of their type I’ve ever used. With Just For Racks you get a bottle of gel, a brush, gloves and a couple of soaking bags. Take the racks and put them in a bag, pour on the gel and seal the bags. Then watch the dirt dissolve. Keep turning the bags every half hour or so, so that the get is evenly spread. I find a couple of hours is more than enough to get them clean but if it’s really bad, give it a bit longer. Any residue left on the racks should be easily removable with a Brillo pad.
The oven cleaner is great too - it can even be used on the glass in the door - just open the door so that the glass is lying flat, brush on and wipe off when the dirt dissolves.