Seems to be quite a few bargains on Play at present, thanks to j17esh for posting the rechargeable battery deal which led me to this.
The Sennheiser CX300 is an old model, but for £5 you can't go wrong.
As these are a 2.5mm jack most people will probably require a 2.5mm to 3.5mm adaptor to use these on most MP3 players.
The cheapest I've found is on Ebay for 99p delivered, UK seller so no need to wait for them to be shipped out from HK. Search for "3.5/3.5mm" in all categories and it should be the second result shown.
Please keep and mind that Play have a habit of not fulfilling orders, so it may pay to wait for these to be dispatched before ordering an adaptor.
EDIT : Just had the majority of the order I placed for the Camlink rechargeable batteries cancelled. I've had 10 out of 16 orders cancelled by Play in the last few months, so it really does pay not to buy any accessories until you have confirmation that your goods are on the way.