Guys - wow, thanks for the fantastic response.

It's definately a willowherb ! (named so because the leaves are similar to the willow tree)...I'm possibly swaying towards the broad-leaved willowherb after some digging on google.

What i'll do is pull the majority out and leave a few in and see how they flower then we can prove conclusively which one it is.

I live on a lane with an overgrown hedgerow opposite where this stuff appears in abundance so I guess the wind has carried the seeds into my garden.

I'll need to fill the gaps I create though - any recommendations for a pretty flowering (colourfull if possible) semi-shade plant ? - sorry for the questions but i'm pretty new to gardening but enjoy helping my garden look its best.

Mike.