OK, I've narrowed it down to
6 pairs of headphones, all of them by Sennheiser!

I'll list them one by one by price with the features they've got listed on the site:
1. Sennheiser PX 40 Mini-Headphones - £11.99 (RRP £14.99)
- Ideal for use with portable audio sources such as MP3 or CD players
- Optimised for rock and pop music
- Excellent sound reproduction with wide dynamics
- Bass tube for rich, full bass
- Ultra-lightweight diaphragm ensures powerful sound
- Cable with convenient volume control
- Lightweight and comfortable for music on the move
- Connection cable made from highly conductive OFC copper
- 2-year guarantee
The PX 40 are open, dynamic, supra-aural mini headphones for portable players. They are fitted with a convenient in-line volume control.
2. Sennheiser HD202 Stereo Headphones - £13.99 (RRP £19.99)
- Closed, supra-aural, dynamic hi-fi stereo headphones
- For DJs, ideal for both mobile sources and home mini hi-fi systems
- Good attenuation of ambient noise
- Earcups can be removed from the headband
- Specially designed damping perforation ensures powerful bass response
- Lightweight diaphragm material with "turbine" embossing for extremely low bass
- Powerful neodymium magnets and lightweight diaphragms for high sound levels
- Powerful bass and increased signal levels for modern rhythm-driven music
- Clips to the belt: cord take-up for adjusting the cable length when listening on the move
- Extremely comfortable to wear due to ultra-lightweight design, even for extended listening
- Rugged outdoor design with extremely flexible headband
- 3m highly conductive OFC copper cable
- Replaceable leatherette ear pads
- 2 year guarantee
The HD 202 closed, dynamic hi-fi stereo headphones are the ideal partner for DJs and powerful modern music, providing good insulation against ambient noise and a vivid, crisp bass response. The rugged lightweight headphones have a secure fit and can be used for both mobile sources and home (mini) hi-fi systems. When out and about, a convenient cord take-up lets you adjust the headphone cable to the required length.
3. Sennheiser PX 100 Mini-Headphones Black - £17.99 (RRP £29.99)
- Dynamic stereo mini headphones
- Great for travel and outdoor use with mobile sources such as CD, MD or MP3 players
- Fold and flip: registered design allows the ear cups to be turned through 90° and fold the headphones closed (headphones will lock in open and closed positions)
- 1.4 m Kevlar-reinforced OFC copper cable with 3.5 mm stereo jack plug
- Fits into the shirt pocket: rugged transport case with cord take-up (145 x 75 x 27 mm)
- Excellent sound quality for mobile audio sources
- High-resolution sound due to Sennheiser "Duofol" diaphragms with spiral embossing (reduces and spreads eigenforms)
- Strong Neodymium magnets for extremely low distortion
- High efficiency and powerful reproduction
- Balanced and detailed sound image for all types of music
- Crisp and fast bass response due to system "twin damping" with 80ppi polyurethane and special perforated elements
- Special baffle (based on the legendary HD 414) ensures very balanced sound
- Lightweight: only 60 g without cable
- Comfortable: headband with split padding
- Rugged and durable: robust steel headband with metal joints, steel-reinforced headband ends, connection to the ear cups tested for durability
- Easily replaceable ear pads and headband padding (ear cups and headband can be replaced by your dealer)
- 2-year guarantee
- Received a 5 star rating in What Hi-Fi Sound And Vision magazine
The PX 100 is a pair of open, dynamic supra-aural mini headphones for mobile players. Their outstanding sound reproduction and foldable design make them ideal for outdoor use and travelling.
4. Sennheiser EH 150 Headphones - £17.99 (RRP £29.99)
- Closed, supra-aural, dynamic hi-fi stereo headphones
- Good attenuation of ambient noise
- Powerful neodymium magnets and lightweight diaphragms for high sound levels
- Extremely comfortable to wear due to ultra-lightweight design, even for extended listening
- Rugged outdoor design with extremely flexible headband
- 3m highly conductive OFC copper cable
- Replaceable leatherette ear pads
- 2 year guarantee
The EH 150 closed, dynamic hi-fi stereo headphones are the ideal partner for powerful modern music, providing good insulation against ambient noise and a vivid, crisp bass response. The rugged lightweight headphones have a secure fit and can be used for both mobile sources and home (mini) hi-fi systems.
5. Sennheiser HD 212 Headphones - £24.99 (RRP £29.99)
- Jack plug 3,5/6,3 mm stereo
- Nominal impedance: 32 Ohm
- Weight w/o cable: Cable 130 g
- Transducer principle: Dynamic, closed
- Ear coupling: Supra-aural
- Cable length: 3 m
- Frequency response (headphones): 12.....19000 Hz
- THD: <0,2 %
- Sound pressure level (SPL): 112 dB(SPL)
The HD 212Pro closed dynamic hi-fi stereo headphones have a balanced sound image with a slight emphasis on the bass response. DJs will like their excellent attenuation of ambient noise and the soft, comfortable ear pads.
6. Sennheiser PX 200 Mini-Headphones Black - £24.99 (RRP £39.99)
- Dynamic stereo mini headphones
- Great for travel and outdoor use with mobile sources such as CD, MD or MP3 players
- Fold and flip: registered design allows the ear cups to be turned through 90° and fold the headphones closed (headphones will lock in open and closed positions)
- 1.4 m Kevlar-reinforced OFC copper cable with 3.5 mm stereo jack plug
- Fits into the shirt pocket: rugged transport case with cord take-up (145 x 75 x 27 mm)
- Ideal for use in coaches, trains etc.: soft ring ear pads with improved fit and closed ear cups ensure excellent passive attenuation of ambient noise (15 to 25 dB for frequencies above
- 1,200 Hz)
- For all types of modern music: detailed sound reproduction with a slight emphasis on the bass
- High-resolution sound due to Sennheiser "Duofol" diaphragms with spiral embossing (reduces and spreads eigenforms)
- Strong Neodymium magnets for extremely low distortion
- High efficiency and powerful reproduction
- Adaptive baffle damping (patent pending)
- Bass tube for powerful, punchy bass
- Lightweight: only 60 g without cable
- Comfortable: headband with split padding
- Rugged and durable: robust steel headband with metal joints, steel-reinforced headband ends, connection to the ear cups tested for durability
- Easily replaceable ear pads and headband padding (ear cups and headband can be replaced by your dealer)
- 2-year guarantee
The closed, dynamic, supra-aural PX 200 mini headphones are an ideal choice for mobile audio sources. Their excellent isolation lets you enjoy music without being disturbed or disturbing others. Folded and packed into their rugged transport case, these headphones will accompany you anywhere.
Personally, the first one I might strike off the shortlist would probably be pair #4, the EH 150s. It's currently got a 3* rating from users on Play.com. I'd then strike off #1, the PX 40s. They haven't been reviewed at all, so I'm not too sure about them.
Still, I'd like to hear your opinions! Would it still be the PX 200s all the way? I think after looking at all of these, I might end up going for the PX 200s. They're the most expensive, but they've got the most praise, and not just from users of Play.com. They're also currently out of stock, so that might be a sign that they're really good!