It's a Euphorbia. I can't tell from the photo how big it is, or what the leaves are like, but it could be the annual weed "sun spurge" - Euphorbia helioscopia.
Remember it's only a weed if you don't like it - otherwise it's a wild flower. ;-)

thanks for the quick reply. will probably be in the garden tomorrow then :)

I agree Euphorbia helioscopia - Wartweed its a weed that gets everywhere and it can irritate the skin when pulled out. I dont let it grow as seeds last for up to 8 years in the ground a real pest.

I would personally remove... with extreme prejudice!

Be sure to not get any of the sap in your eyes. It can cause temporary blindness. And take care near the fish pond, if you have one. It will kill fish (I think).

Like lots of other highly poisonous plants its active ingredients are extracted and used in the pharmaceutical industry. A lot of animals and humans with diseases such as cancer and heart disease have every reason to be very grateful that plants like this exists and that research has uncovered their secrets.

Pull it out quick before it flowers or you will have it for ever.