That is a good question and one I personally would like the answer to. Sorry not to have been more help but I feel sure some one will tell us.

Could it be hairy bitter cress? Bit difficult to tell from the photo. Leave one in until it flowers and then it should be easy to identify.
If you can't wait for that, take a look at this site which has some good pictures of leaves of common weeds.
http://www.dgsgardening.btinternet.co.uk/weedlf.htm

I do not know the name either but they are self seeding annuals. I have been weeding them out this am. I'd get rid, as one years weeds are seven years seeds. They are easily pulled out as they are just seedlings.

Do the leaves have a purple tinge?

I'd agree with Bertie looks like a bittercress...

Thanks for the feedback folks. I've compared an actual leaf with the samples on the excellent site suggested by Bertiefox and the nearest match seems to be Nipplewort. I'll wait til it flowers and then repeat the question if that's OK.

thanks bertie for the site suggestion. i have finally ID the most problomatic weed in my garden(figwort)

Interested to hear how Figwort is one of the worst weeds in your garden. I remember years ago how I found what I thought was this attractive little plant with its small tubular flowers, which wasps liked visiting.
I convinced myself it was some precious rare flower, and accordingly proceeded to propagate it everywhere.
You can guess the result.
Nowadays I'd never get caught like that as I check up on every thing that I'm not sure about. And with both Japanese Knotweed, and Horsetail here, we have plenty to keep us occupied without introducing any new interesting plants.