It could be bind weed does it have white flowers.

I think this is the annual Black Bindweed which is a relative of Persicaria. Pull it out before it seeds. The saving grace is that it's an annual and doesn't have the creeping perennial roots of the unrelated common Bindweed.

I can see the papery stipule at the base of the leaf in the first pic which is a give away.

Yes it is a Bindweed variety, I would pull it out as soon as you can. I am on my third year of fighting the common bindweed that comes over from my neighbours, fingers crossed yours it the annual one and you can get rid of!

Thanks guys. The worry is that I grew it from seed from a packet I bought at Tesco which said it had blue cornflower seeds in. I am new at this! :o)