Sorry about the late reply. I basically chose to ignore all the negative advice and get myself one of the ultimate cycle masks. I do get one or two odd looks but I SWEAR it is the best cycle accessory I have now. I have been using it for a couple of weeks now and I can't recommend it enough.
To answer some of the negative questions:
Restrict breathing: yeah maybe but after a couple of days you don't feel any restriction.
Comfort: the mask looks big but weighs next to nothing and fits nicely under my helmet.
Car users getting more pollution: not sure I agree. when I drive a car I hit the internal airflow button and i don't breathe any air from outside until i'm out in the countryside.
Oh also, I don't get a sore throat after cycling on really traffic heavy days!
Thanks for all the positive support from those (few) who gave it. Your lungs ARE precious, and it's worth remembering that...
EDIT: I just read that bbc article, yeah that's why i wasn't sure about going for the ones that they sell in regular cycle shops. figured 3M would make a decent one that actually passes various EU tests etc.