These look a little like Southern Marsh Orchids. (In fact I thought they were something like that, before a kind botanist on the WAB site identified them for me).
They are Marsh Woundwort, a species of plant in the "Mint family", common around marshes, rivers and streams all over the UK, click on the photos to enlarge them.
Their leaves have been used in the treatment of inflamed spots and to stem bleeding since ancient times.
The hoverfly on the right of the Woundwort is Rhingia campestris, easily recognised by its long upturned 'snout' and orange abdomen. The Rhingia hoverflies have amazingly long tongues, are the real pollen specialists amongst hoverflies, and the larvae are often born in cow dung.
The yellow flowers behind the Woundwort belong to the very abundant (along the Thames), "Yellow Loosestrife" (part of the "Primrose family").
These, like Woundwort, can often be found growing around ponds, rivers etc...
Once again, these plants can be used as a remedy for diaorrhea and even dysentry.
Their bright yellow flowers make a decent yellow dye, their brown, underground rhizomes make a decent brown die, and the growing plants are effective insect repellents. They were often burned in houses to drive away flies.
NB. I have (at last) identified the flower in our garden that I didn't know, (in the garden flowers post a few days ago). Its called Redshank (I know not why, as unlike the wading bird of the same name, which is very aptly named - having red legs, this flower neither has legs or red legs?!), and is not particularly jaw-dropping in terms of its interest to most botanists.
Aw well. At least its got a name now!