http://www.tradingstandards.gov.uk
You do have rights as a consumer and Apple can't ignore that. the crack on top is obviously from poor design on Apple's part. Not considering that when you use magnets there will quite a bit of pressure on that particular area over time they basically set it up to crack. As anybody can see, there is no support underneath so that crack would be fairly self-explanatory to anybody looking at this objectively. Since there are quite a few people who have similar cracks, you're not alone with this problem and that;s always good.
I would start with Apple customer relations UK and then move on to the above site - call them for advice on how to deal with Apple. When I lived in England I did and they were exceedingly helpful.
Anyway, good luck!
PDE why do you own Apple products at all? You are Bashing the "poor design" here, and you are on a crusade againt the MBP screen yellowing. Im a skeptical person by nature, but you make me look like a complete polyanna comparatively.