We managed to get £15 from a £3 tub of pate once. It had a bit of blue plastic in it, so we too it back to the supermarket, who refunded the price of the product.
We then got a letter from them a couple of weeks later saying that they had brought it up with the manufacturer, and enclosed vouchers as a good will gesture.
We then got another letter from the makers of the pate apologising for the problem, and they enclosed a (wait for it) a postal order for even more money.
The shop in question was Somerfield, and the pate was from Cauldron Foods. We were quite happy that the shop had refunded the cost of the product. In all, a good return on what was a bit of the mixing machine that fell into the pate before it was potted up.
Complaining does work, and if you do it calmly and politely, you can get a lot out of it.