It looks very much like Himalan Balsam, presumably a bit late in the season for many flowers. It is classifed as an alien invasive weed and thrives in moist places.

Now that a clever person has answered ..
I just want to say... Hi Mike :o))))))))

I agree with Bulbaholic. It looks like Impatiens glandulifera, Himalyan balsam.

Another clever person :o)

Thank you people- pulled enough of the stuff out so should have known! And Hi Terratoonie!

Assuming this is in your garden and not on a river bank I'd go for the Roundup myself - and I do not like using chemicals but this is a pain to get rid of

I'm sure that isn't Himalayan Balsam. I'm sorry to throw a spanner in the works, but we have loads of the stuff growing around the Lake District, and the National Trust are trying to get rid of it, as it is such a pest. I've had it in the garden, and it doesn't look like your plant, Mike. The flowers are larger than yours, and they hang under the leaves on a longer stem. Also, the leaves are too leathery to be Balsam. I suggest you look up Himalayan Balsam on the net, and ID it that way. It may be a much more interesting plant that you don't really want to get rid of, and all it needs is a trim.
You have a very interesting garden, bye the way, full of surprises by the look of it....Annie

http://images.search.yahoo.com/search/images;_ylt=A0oG7m1tN25OJDAAlTFXNyoA?p=himalayan+balsam&fr2=piv-web

plenty to check ..

I also have my doubts as it looks nothing like the HB that grows in my garden. The leaves dont look toothed enough. The stems are hollow and have clear obvious nodes and these dont.

any chance of a close up of the flower.

I also agree with Anne, this is not H.B. the stems are not fleshy enough, HB stems are almost like busy lizzy stems in fact it is from the same family.

Interesting- I too have pulled up much HB at an orchard I've been working at and it looked nothing like this stuff! I'ts more shrubby. No Moon_grower ,its not my garden but some council owned land we've been given permission to create a forest garden in. Actually I wouldn't use round-up on anything, the more I learn about Monsanto ,the less inclined I am to give them any of my money.
Will investigate further......

OK, so you guys are saying that it is not Himalayan Balsam, but no one is suggesting an alternative! It is growing as a weed and is not a British native that I know, therfore an alien garden escapee.
I have had my shot, now what is yours ;-)

all I can find similar is a sesame