Well hopefully they are just one off's and not becoming the norm. For as long as they have been selling at £50, it's only the last week or so that people have been mentioning issues. Could be just a bad batch that were made. You will find no other headphones will sound as good up to £100, and they can be returned should there be any issue.
Look at Sennheiser's HD555 headphones. They shoved the price from £50-£60, up to £80, and did nothing to resolve the design flaw that results in the plastic cracking above the ear cups. It's very possible that they were just bad batches, but none the less, it happens even to well respected headphone manufacturers such as Sennheiser.