For me this is quite difficult territory, the world of the Fabaceae or what used to be called the Leguminosae. This family include the peas, so if you know what a pea looks like you should be able to tell if the plant is in the Fabaceae. Rose (2006) has a useful diagram on page 266 labelling the various parts of the pea flower. The top most petal which often stands erect is the standard. Immediately beneath it are 2 wing petals and at the bottom are 2 petals fused together into one called the keel.
This photo shows the keel petal to the right, all the other petals have been removed. Note how well it hides the stigma and stamens. On the left is the calyx which forms a tube with teeth, these teeth are longer at the bottom of the flower, and this is one of the diagnostic features that separates V sepium from V sativa.( the latter has more even teeth length).
Concealed by the keel petals are the stamens (10) and style (1). The stamens are joined to form a tube.
So coming back to my difficulty with Fabaceae, I think its because its easy to confuse say V sepium and V sativa and the genera of Vicia and Lathyrus are very close, as are Medicago lupulina and Trifolium campestre. For example with Rose(2006) the couplet key for V sepium and V sativa is as follows(p281)
Plant per- calyx teeth unequal, lower usually longer than upper- lfts generally widest below middle- fls dull purple, usually more than 2 together……V sepium
Plant ann- calyx teeth almost equal- lfts generally widest at or above middle- fls pink or purple, usually only 1-3 together………………………………..V sativa
So there are lots of use of the words; usually and generally. Calyx teeth length is the feature used to differentiate the 2 species in Streeter (2010), so all in all, I think this is Bush Vetch Vicia sepium.