Help with identifying a plant (or possibly weed)

Hello there,

*total newb question follows*

My wife and I have just moved into our first 'proper' house, complete with our first garden. When we moved in the garden was just wild, with weeds over 6ft tall. We've since had the garden flattened, rotavated and planted grass seed.

Over the last month or so the weather has gotten a fair bit warmer, with 2-3 weeks of dry period followed by rain over the last week. This rainier period has coincided with the arrival of the plant which is in the attached photo, which is appearing everywhere...

I've spent some time looking on google but can't see anything which will help me identify it. At the rate it's growing, I think it's a weed but really can't be sure.

Any help identifying the plant, and if necessary offering advice on how to treat it, would be appreciated. It's giving my wife the 'heebie-jeebies' (it's making her skin crawl).

Thanks!