Buckthorn (Rhamnus cathartica)
Buckthorn is a dense deciduous shrub or small tree which thrives on chalk soils. It has large thorns on the twigs. The fruit is a berry, which is green and then a glossy, purplish-black when ripe. If eaten the berries have a laxative effect, from where it gets the common name of Purging Buckthorn.
You can find more information about the wild flowers and other plants found on Magog Down in our Flowers throughout the Year critterdex.