You're trying to do two things with one microphone.
The phrase "jack of all trades, master of none" applies to microphones as well! I would suggest that you go for a good hypercardiod microphone rather than a "general purpose" video microphone such as the 300 (which is over-rated in my opinion).
If you make £100 your limit, then you might as well stay with the mic on your camera. You ain't gonna get anything decent for that price. The Audio Technica ATM33A is not bad tho'
Think seriously about the K6 series from Sennheiser, or something from Audio Technica such as the AT815 B or the AT897. If you're willing to go to the £300 mark, then you start to get in the range of some seriously good value for money microphones.
http://www.sennheiser.co.uk/uk/icm.n...microphones_k6 http://www.audio-technica.com/cgi-bi...nes%3A+Shotgun
Edit: I know microphones aren't as "sexy" as some video gear and the prices can seem a bit daunting. I also realise that cost is a major factor but buying a cheap microphone really is a waste of money. Far better to wait a bit and get something where you'll really notice the upgrade.