Hemlock is not the common species it once was, especially in farming areas like Galloway. It is as toxic to livestock as it is to humans, so farmers have all but eliminated it from inland areas. It is most commonly found coastally, having a preference for sandy soils, but that’s not to say it doesn’t lurk inland too. It is more common in E Scotland and S England.
Though up to 2m tall, it has more delicate looking (fern-like) leaves and white flower umbels than many of its carrot family relatives (cow parsely is probably most similar). The key diagnostic is its hollow, cylindrical, red spotted stems. It has an unpleasant mousy smell.
Coniine is the main active toxin which attacks the central nervous system, leading to paralysis and subsequent asphyxiation. 8 – 10 leaves could send you to your maker, though there is much variation according to season, stage of growth, location etc. The toxins are more concentrated in the roots and seeds.
For more information on this and other members of the carrot family see “Know Your Carrots” and my in depth article – “An Introduction to the Apiaceae Family for Foragers”
For more discussion on toxic plants and fungi see “The Day I Ate a Deadly Plant: The Spectrum of Edibility”