It's a euphorbia - a caper spurge.
"Common Name CAPER SPURGE
Latin binomial Euphorbia lathyris
Plant family Euphorbiaceae
Names in other languages euphorbe epurge (F), kreuzblattrige wolfsmilch (D), leche trezna (E)
Caper spurge is often known as the 'mole plant' because of its reputation for deterring the activity of this creature. A native of Mediterranean areas, it has spread into Northern Europe, and has also been introduced into the USA. The seeds have in the past been used for medicinal purposes and for the production of lamp oil; the oil content of the seeds is high, about 50 percent by weight, of which some 85 percent is in the form of oleic acid, used in the preparation of some soaps, detergents, lubricants, paints and cosmetics: the oil has also been suggested as a possible diesel fuel substitute. Research on the crop has been conducted in the USA and especially Germany, where yields of over 2 t seed per ha have been achieved from experimental plots; small trials in the United Kingdom have indicated that similarly high yields may be obtainable elsewhere. "
Source:hxxp://www.biomatnet.org/secure/Crops/S598.htm
And it is flowering - not much of a flower is it? The sap is white, and can make your skin burn.