cleaning enamel oven trays

Agapanthus

Far and away the best thing that I have ever tried for this is Ovenmate from Lakeland. You just paint it on (there's a brush in the box) and wash it off after a few hours. Like most things from Lakeland it's not cheap, but it does a brilliant job and a lot less damage to the enamel than scourers.

If the budget doesn't run to that, then any sort of oven cleaner will do a fair job. Oven cleaner is basically caustic soda, so you can use that too, but it's nasty stuff to handle - rubber gloves only!

bluebag

Bio washing powder and a drop of bleach in as hot a water as you can muster and soak overnight.

Ozne

Just do the ammonia thing with the bag. That will take off stuff that's been burnt on for years, so they should come up like new.