Caustic soda is the way. Big bowl of warm (not hot, or the reaction goes a little crazy - think all over your face, I learnt from bad experience and a week of red spots all over the face!), add a table spoon of soda and stir. Now lower item in to bowl and stir, remove after 10/15 seconds and wash with cold water. Repeat the dunk and wash until everything is off, now brasso to hearts content. Don't just dunk the item in and leave it for a long time as the soda will badly pit the ali and then the finish looks like sh*t. Little and often is the way.
Tescos in the cleaning section (caustic soda is a great drain clearer and fat remover) or B&Q. No more than $5 a pot and that will last you a long while