Following a period of inactivity, a positive and lasting influence on his work arose from his meeting in 1916 with the Dutch composer Bernard van Dieren; he also gained creative impetus from a year spent in Ireland, studying Celtic culture and language.
On his return to England in 1918, Heseltine began composing songs in a distinctive, original style, while building a reputation as a combative and controversial music critic.
The work made little impression on him until he discovered that his uncle, Arthur Joseph Heseltine (known as "Joe"), an artist, lived close to Delius's home in Grez-sur-Loing in France.
He also found a kindred spirit in an Eton music teacher and Delius advocate, the cellist Edward Mason, from whom Heseltine borrowed a copy of the score of Sea Drift.
[10] In June 1911 Heseltine learned that Thomas Beecham was to conduct an all-Delius concert at London's Queen's Hall on the 16th of that month, at which the composer would be present, and his Songs of Sunset would be given its first performance.
[21] A female acquaintance at Christ Church described the 19-year-old Heseltine as "probably about 22, but he appears to be years older ... 6 feet high, absolutely fit ... brilliant blue eyes ... and the curved lips and highhead carriage of a young Greek God".
In April 1914 he spent part of his Easter vacation with Delius at Grez, and worked with the composer on the scores of An Arabesque and Fennimore and Gerda, in the latter case providing an English version of the libretto.
[26] However, his life as a student in London was brief; in February 1915, with the help of Lady Emerald Cunard (a mistress of Beecham) he secured a job as a music critic for the Daily Mail at a salary of £100 per year.
[13] Heseltine spent much of the 1915 summer in a rented holiday cottage in the Vale of Evesham, with a party that included a young artist's model named Minnie Lucie Channing, who was known as "Puma" because of her volatile temperament.
By the summer of 1917, as Allied fortunes in the war stagnated, Heseltine's military exemption came under review; to forestall a possible conscription, in August 1917 he moved to Ireland, taking Puma, with whom he had decided he was, after all, in love.
[47] In Ireland Heseltine combined studies of early music with a fascination for Celtic languages, withdrawing for a two-month period to a remote island where Irish was spoken exclusively.
[50] The composer Denis ApIvor has indicated that Heseltine's obsession with the occult was eventually replaced by his studies in religious philosophies, to which he was drawn through membership of a theosophist group in Dublin.
[4][13] At around this time the composer Charles Wilfred Orr recalled Heseltine as "a tall fair youth of about my own age", trying without success to convince a sceptical Delius of the merits of Van Dieren's piano works.
[65] The Welsh years were marked by intense creative compositional and literary activity; some of Heseltine's best-known music, including the song-cycles Lilligay and The Curlew, were completed along with numerous songs, choral settings, and a string serenade composed to honour Delius's 60th birthday in 1922.
[76] In June 1924 Heseltine left Cefn Bryntalch and lived briefly in a Chelsea flat, a stay marked by wild parties and considerable damage to the property.
[78] The other permanent Eynsford residents were Barbara Peache, Heseltine's long-term girlfriend whom he had known since the early 1920s, and Hal Collins, a New Zealand Māori who acted as a general factotum.
He sought concert reviewing and cataloguing assignments without much success; his main creative activity was the editing, under the pseudonym "Rab Noolas" ("Saloon Bar" backwards), of Merry-Go-Down, an anthology in praise of drinking.
With no fresh creative inspiration, he worked in the British Museum to transcribe the music of English composer Cipriani Potter, and made a solo version of "Bethlehem Down" with organ accompaniment.
[103] Most commentators have considered suicide the more likely cause; Heseltine's close friend Lionel Jellinek and Peache both recalled that he had previously threatened to take his life by gas and the outline of a new will was found among the papers in the flat.
[118] In the early 20th century the German-influenced 19th-century song-writing traditions generally followed by Hubert Parry, Charles Villiers Stanford, Edward Elgar and Roger Quilter, were in a process of eclipse.
For composers such as Ralph Vaughan Williams and George Butterworth, English folk-song became a dominant feature of their work;[119] at the same time, songwriters were seeking to extend their art by moving beyond the piano to develop richer forms of vocal accompaniment.
[122] Gray writes that from Van Dieren, Heseltine "learned to purify and organise his harmonic texture ... and the thick, muddy chords which characterised the early songs gave place to clear and vigorous part-writing".
[13][122] Gray summarised this style thus; They [the differing elements] are fused together in a curiously personal way: the separate ingredients can be analysed and defined, but not the ultimate product, which is not Dowland plus Van Dieren or Elizabethan plus modern, but simply something wholly individual and unanalysable—Peter Warlock.
[4][13] In a summary of the Warlock oeuvre, Copley asserts that Heseltine was a natural melodist in the Schubert mould: "With very few exceptions his melodies will stand on their own ... they can be sung by themselves with no accompaniment, as complete and satisfying as folk-songs".
[127] Copley identifies certain characteristic motifs or "fingerprints", which recur throughout the works and which are used to depict differences of mood and atmosphere: anguish, resignation but also warmth, tenderness and amorous dalliance.
[128] The music critic Ernest Bradbury comments that Heseltine's songs "serve both singer and poet, the one in their memorably tuneful vocal lines, the other in a scrupulous regard for correct accentuation free from any suggestion of pedantry".
[122] In musical parlance Heseltine was a miniaturist, a title which he was happy to accept in disregard of the sometimes derogatory implications of the label: "I have neither the impulse nor the ability to erect monuments before which a new generation will bow down".
[133] In 1922, in the same magazine, the short song cycle Mr Belloc's Fancy was likewise praised, especially "Warlock's rattling good tunes and appropriately full-blooded accompaniment".
[135] Early in 1925 the BBC broadcast a performance of the Serenade for string orchestra written to honour Delius, a sign, says Smith, that the music establishment was beginning to take Warlock seriously.
[122] Collins acknowledges that the Warlock output includes much that can be dismissed as mere programme-fillers and encore items, but these do not detract from numerous works of the highest quality, "frequently thrilling and passionate and, occasionally, innovative to the point of being revolutionary".