The CX300 would be one of the last earphones i'd go for, for the simple reason of there being so many fakes. and the fakes are becoming far more sophisticated.
http://headphonedeals.org/fake-headphones/
second there are some good choices out there for you. with a slight sacrifice of bass, but with a lot more detail and clarity, you could have the CrossRoads Mylar X3's
http://www.jaben.net/shopping.html
if you could go to £40 or have a look on ebay, then try for a pair Metro.fi 2's. my Goldring GX200's are speeding their way to me, but i've yet to hear a bad report about any Goldring headphones.
just be careful on the CX300's. i always say buy from amazon themselves.