Heseltine wrote his earliest mature compositions, published to critical acclaim under the newly adopted pseudonym Peter Warlock, following his sojourn in Ireland of 1917-1918. They were followed by a period of concentration on musical journalism; for a while, he was the editor of the musical magazine The Sackbut. His most prolific period, both as a composer and author, was in the early 1920s when he withdrew from the financial and social pressures of London to his mother's and stepfather's house, "Cefn Bryntalch", in Montgomeryshire, mid-Wales, where he wrote some of his finest songs, finally completing his song-cycle The Curlew to poems by W. B. Yeats. During this period he also met Bartók, who visited him while returning from a concert in Aberystwyth arranged by Professor Walford Davies, and whose influence can perhaps be seen in The Curlew.
Between 1925 and 1929, following a quiet period, Warlock and his colleague E. J. Moeran led a wild, boozy life in Eynsford, Kent, having to deal with the local police more than once. For Warlock, however, this was one of the most fruitful periods of his life, but by the end of the 1920s his creativity was on the decrease and he had to support himself with music criticism again. He was suffering from severe depression, but whether his death from gas poisoning at the age of 36 was suicide or an accident is not known for certain. His cat had been put out of the room before he died, perhaps to spare it. There is a third possibility: Warlock had made Bernard van Dieren his heir in his will, inspiring claims by Warlock's son Nigel Heseltine that van Dieren had murdered his father.
An intriguing figure, Warlock has served to inspire several characters in English-language literature, among them: Coleman in Aldous Huxley's Antic Hay (1923), Roy Hartle in Osbert Sitwell's Those Were the Days (1938), Giles Revelstoke in Robertson Davies' A Mixture of Frailties (1958) and Maclintick in Casanova's Chinese Restaurant (1960) by Anthony Powell. D. H. Lawrence's use of Warlock as the model for Julius Halliday in novel Women in Love (1920) led to a threat of a lawsuit, followed by an out of court settlement. His name is surrounded by rumours of involvement with the occult, an interest which he shared with others in the bohemian world of the early 20th century - for example the novelist Mary Butts asserted that it was Warlock who initially introduced her to these subjects. Other less conventional aspects of Peter Warlock's life include experimentation with cannabis tincture, a gift for the composition of obscene limericks and a marked interest in flagellation.
He wrote little instrumental music, although the Capriol (October 1926) is probably his best-known work and exists in versions for string orchestra, full orchestra and piano duet. (There are arrangements for other combinations but these are not by Warlock himself.) His only composition for solo piano is a set of arrangements of Celtic melodies, the "Folk-song preludes". He had a deep affinity for poetry, especially that of Yeats and his friends Robert Nichols and Bruce Blunt (1899-1957), and he always chose texts of high artistic value, many of them from the Middle Ages, as basis for his songs.
Many people consider his greatest work to be the song-cycle "The Curlew", for tenor and chamber ensemble, in which he sets four linked poems by Yeats. It is certainly his most substantial piece and was written over a long period of time - some seven years - taking in many stylistic changes along the way from the neo-Delianism of "The lover mourns for the loss of love" to sections within the longest song, "The withering of the boughs" that suggest Bartók and Schoenberg as influences before achieving a more idiosyncratic, modal, and genuinely Warlockian vocabulary.
Warlock's musical tastes were wide, from Renaissance music to Bartók. In his own works, we hear a development from emulation of the Victorian and Edwardian drawing-room style to a more contrapuntal, strongly personal idiom characterised by the relationship between modal lines and a distinctive palette of chords. He was unusual amongst composers of his generation in being largely unaffected by the folksong movement, either as an arranger (the above-named piano pieces being an exception) or a composer. He only wrote one folksong-oriented work, the cycle "Lilligay" and it might be more appropriate to look to Bartók as an influence here rather than any paradigms from his own country.
Apart from original works, Warlock edited and transcribed many lute songs by Elizabethan and Jacobean composers in addition to music by Purcell and other Baroque composers. He also did much to promote the music of Delius, especially by organizing the successful Delius Festival of 1929 with Thomas Beecham. He wrote the first biography of Delius as well as, with Cecil Gray, a book about Carlo Gesualdo. His book on "The English ayre" was a groundbreaking study but he also wrote about contemporary music including an article that was probably the first substantial study in English of the music of Arnold Schoenberg. In 1925, Warlock rediscovered the music of sixteenth century composer Thomas Whythorne, releasing a book of his compositions and poetry.
Warlock also edited, under the pseudonym 'Rab Noolas' (to be read backwards), an anthology on drinking 'for the delectation of serious topers', entitled 'Merry-Go-Down' (Mandrake Press, c. 1930).