I've seen something very similar in the Midlands and couldn't find an ID either. It was also growing on wet grassland. I have some photos somewhere that I'll (eventually) get around to putting on iSpot.

Yellow loosestrife. I'll be quite chuffed if I got this one right! It is found in gardens I think (possibly a variety), but it is native too.

I was put off Yellow Loosestrife (Lysimachia vulgaris) initially because the flowers were usually shown branching from the main stem (e.g. http://www.floralimages.co.uk/plysimvulga.htm), but the plant I'd seen the flowers were always very close to the stem. I notice there's the non-native Dotted Loosestrife (Lysimachia punctata) which could fit the bill (see http://www.floralimages.co.uk/plysimpunct.htm). The latter better fits the plant I saw (from memory - I'll have to dig out the photos now).

This is a frequent and vigorous garden escape

This looks more like Dotted Loosestrife, L. punctata, it can be recognised by the shortly petiolate leaves (more or less sessile in L. vulgaris) ridged stems and the fringed petal margins.

I have added two photographs, close-up of the flower and a view looking down the stem to a leaf and flower join.
does that help?

Pliny the Elder in his "Naturalis Historia" claims the name comes from the act of putting loosestrife in the yokes of skittish ox when ploughing - they loose the 'strife' caused by being yolked. He also claims if you burn loosestrife you will not be bothered by snakes or flies.