He is particularly known for his studies of the butterfly Apatura iris (purple emperor), and for his discovery of the Nigerian subspecies of the pygmy hippopotamus, named Choeropsis liberiensis heslopi after him.

Heslop entered the Colonial Service in 1929 and became an administrator in the Owerri province of Nigeria, where he became a prolific hunter and documented what may be the last reliable sightings of Choeropsis liberiensis heslopi.

He wrote most of Notes and Views of the Purple Emperor, a 1964 collection of papers on Apatura iris, which Oates has called "a meditation on [his] all-pervading passion".

[6] According to Oates's biography, the younger Heslop first began to collect butterflies at the age of seven, when a family member gave him a jar of Pieris larvae to keep him entertained through a bout of mumps.

[5] On his eleventh birthday, he collected his first Vanessa cardui (painted lady) using a net owned by his uncle, a vicar, who used it to catch bats in his church.

[11] He met and befriended Charles de Worms, then studying at King's College, who would become a noted lepidopterologist and Heslop's long-time friend and collecting companion.

[16] His duties in Nigeria included presiding over legal cases: he acted as a magistrate in trials of local people charged with slave trading, and was once required to supervise an execution.

[6] Before the 1940s, western naturalists generally considered that the animals could not be found in Nigeria, since the nearest generally-recognised population was in Liberia, around 1,000 miles (1,600 km) to the west.

[18] He counted the skull and jaw of his third hippopotamus among his most prized possessions, and kept them until his death, when his wife Eileen donated them to the British Museum along with his hunting records.

[16] In 1953, the Nigerian Inspector-General of Forests acknowledged the pygmy hippopotamus as among Nigeria's native mammals, albeit as the one with the narrowest known geographical range.

[5] He collected his first example of Leptidea sinapis (wood white) on the tracks at a railway station, only narrowly escaping being hit by an oncoming express train.

In 1968, aged sixty-four, he waded and swam into a flooded Woodwalton Fen to collect examples of Lycaena dispar batavus (large copper).

[33] He captured his first specimen of Apatura iris at Fox Hill, near Petworth in West Sussex, in July 1935, having narrowly failed to catch three during an expedition with De Worms and a Colonel Labouchere at Bignor in 1933.

[6][d] Heslop published Notes and Views, originally intended as a guide to collecting Apatura iris, in 1964, alongside the naturalists George E. Hyde and Roy E. Stockley.

It comprises a collection of thirty-three papers, often reprints from academic journals, of which twenty-nine were written by Heslop, three by Stockley and one by Hyde, who also provided the book's colour photographs.

[16] These schools were located in Wiltshire, Sussex, in the Cotswolds[42] and near Romsey in Hampshire, all – "providentially", as de Worms put it in his obituary of Heslop – near rich habitats of Apatura iris and other collectable butterflies.

[43] Heslop was involved in the establishment of nature reserves around the United Kingdom, including Blackmoor Copse near Salisbury and Shapwick Heath in Somerset.

[45] He raised money to purchase 46 acres (19 ha) of Blackmoor Copse in 1956, counteracting a plan to replace most of its trees with conifers, and administered it as a reserve for Apatura iris.

[46] Towards the end of his life, Heslop was a frequent panellist on Country Parliament,[12] a BBC Radio 4 show which answered listeners' questions on wildlife and the countryside.