Can someone identify this for me?

I've seen this growing wild, which doesn't necessarily mean it's native, but I thought this was the right section. The people who owned our house before us planted it around the pond (which is terrible, as it chucks tonnes of dead flowers in) and it's (so far) unkillable, but I can't figure out what it is.

 

Hi Dragonrat (good name). I think it is yellow loosestrife, Lysimachia vulgaris.

http://www.ct-botanical-society.org/galleries/lysimachiavulg.html

 

That's given me a good starting point. I think it's L. Punctata rather than Vulgaris - the flowers are positioned differently on the plant.