There are a couple of possibilities of what this might be:
1) When we moved in (Oct 2010) there were thousands of tiny black midgie-like flies living in the light fittings (the people who had the house before us were proper scumbags and our landlords are just crap), however I only got one bite from October to January. These evil things have successfully killed all the squash/melon I've started off as seedlings inside (they laid their larvae in the soil and I guess the larvae ate the roots). But then, with regards to biting, why would they only bite me on the legs when there's a whole scrumptious body they could feast on (they can fly after all).
2) Second possible culprit is horse fly, out the back of the garden is a horse field, so it's possible. But then there have been days I haven't been out there and I've still been bitten. Plus I neurotically check my legs for any beasties now and they're pretty noticeable, haven't seen any.
3) Last culprit is something from my flatmates cupboard ecosystem, last time I went in there I found a load of teeny tiny things which looked like mites. Not entirely sure what they were but I found one had made it into the bathroom yesterday

The weird bit is I'm the only one who is getting eaten, bed bugs love G, so if we had them he would have been the first to get bitten. They're really horrible bites, either swollen and dark like a bruise or a massive hole in the skin! We don't have animals, so probably not fleas

I could post a picture of my legs if that would help?