Soda crystals, try liquid ones in a spray. Spray liberally all over, leave for 20 minutes, then spray again and scrub or sponge down where you can. Then go over it with a sponge soaked in clean water to avoid white marks from the dried up soda.
Cilit Bang is the best for oven cleans.
Just spray on, leave for half hour and, with wire wool, use a little elbow grease! Wipe clean with a cloth and ta-dah... sparkly oven!