have a look at these - they are superb and no more expensive than 'brand name' filters....not just polarisers, but various NDs as well.
PS - just read about your budget! You will be hard pressed to get a good circ pol for £50. I have a spare Hoya Pro 1 you could have for £75 (77mm) If you don't need a 77mm, then I can't help, all my lenses are 77mm thread!
If you look on e6ay and see new ones for silly money - ask yourself why they are £40 and Wex need £115 for the same thing. The answer is, IT ISN'T THE SAME THING....there are manyt fakes out there. caveat Emptor! Don't be suckered into thinking you will get a top quality polariser for £40. Many fall for it - and continue to fall for these copies.