[2] Williams claimed that "I tossed it off (i.e. the sketch of it) in an evening" – in a letter to Idris Lewis (musical director of British International Pictures)[3] in June 1942.
[e][15] The trumpet, now muted, introduces Deryn y Bwn (the rhythm of the original tune is adapted to match that of Jim Cro).
[f][12][18] The slower middle section, marked molto tranquillo (very calmly), begins with Si lwli mabi on flute;[19] followed by Gee Geffyl Bach on french horn;[20] and ends with Cysga Di, Fy Mhlentyn Tlws (on oboe and bassoon).
[21] The last section returns to the lively pace of the opening with a reprise of Jim Cro before Yr eneth ffein ddu is introduced on oboe and violins.
[29] A commercially successful recording of Fantasia (with works by other Welsh composers) by the NYOW followed in 1969 and sold 20,000 copies in less than a year.
[33] Guy and Llewelyn-Jones note that the use of percussion emphasises the "colours and imagery of childhood" and the harp "adds some strong Celtic flavour".
[34] Cotterill notes that "The Fantasia's greatest achievement, however, in spite of its necessarily fragmented nature and thematic diversity, is that it retains a sophisticated level of cohesion that Hen Walia [an earlier work] fails to approach by more confidently deploying its traditional material.
[g][42] In 1968 the National Youth Orchestra of Wales ran a competition to attract young composers but failed to find any suitable compositions, so planned to return to the Fantasia.