Thallium was discovered by Sir William Crookes in 1861. It is a ‘heavy metal’, in the form of salt, closely related to mercury and lead, but much more toxic. Historically, it has been used in pesticides.

Victims of thallium poisoning invariably lose their hair, and complain of extreme pain in body extremities, – fingers and toes – and leg pains so severe the weight of bed clothes alone can cause the victim to cry out in agony.

Strangely, thallium has rarely been used by poisoners, despite being colourless, tasteless and easily dissolvable in liquids. Symptoms of thallium poisoning can be confused with other, common ailments, making it the ideal drug for murder.

Young’s diary was discovered by police under his bed at his Hemel Hempstead bedsit. It was entitled ‘A Student’s and Officer’s Casebook’. It was hand-written in loose-leaf pages, with the names of victims denoted by their initials.

Of Fred Biggs, ‘F’ in the diary, Young wrote: ‘I have administered a fatal dose of the special compound… it seems a shame to condemn such a likeable man to such a horrible end… he is doomed to premature decease… it will remove one more casualty from the crowded field of battle’.

Another entry read:

‘It looks like I might be detected… I shall have to destroy myself’.

Some of Young’s victims were given the lesser substance, antimony, a toxic metallic element. It made them ill, but they survived.