In reply to EuanM:
Grivel Spiders are definitely not what you want. I'd say they are pavement oriented, or for walking where you can place your whole foot down, rather than heel striking.
Personally I've run in a variety of snow and ice conditions on the Lakeland fells in Kahtoola Microspikes and Nanospikes. I think the Microspikes are the best tool for the job. I've not pushed them too hard and there's terrain I'd rather avoid as they aren't designed for proper steep ground, but if it's vaguely a footpath (not a scramble) then so far they've been great. Best thing is how quick and easy they go on and come off. I know people who leave them on for long sections of non-snow and they've worn the spikes down so had to buy a new pair. They take 5 seconds to whip off, and 20 to pull back on. Also, I'm yet to meet anyone who's had one fall off, unlike Yaktraks.
The Nanospikes by comparison are lighter and more packable, but not as good in snow, and make a noise which goes right through me on tarmac. Great on ice, and hard packed snow.