Question: What is the last name of the person whose preliminary work came to a halt in April 1948?  Answer the above question based on the context below:  In 1947, Walton was presented with the Royal Philharmonic Society's Gold Medal. In the same year he accepted an invitation from the BBC to compose his first opera. He decided to base it on Chaucer's Troilus and Criseyde, but his preliminary work came to a halt in April 1948 when Alice Wimborne died. To take Walton's mind off his grief, the music publisher Leslie Boosey persuaded him to be a British delegate to a conference on copyright in Buenos Aires later that year.  While there, Walton met Susana Gil Passo (1926–2010), daughter of an Argentine lawyer. At 22 she was 24 years younger than Walton (Alice Wimborne had been 22 years his senior), and at first she ridiculed his romantic interest in her. He persisted, and she eventually accepted his proposal of marriage. The wedding was held in Buenos Aires in December 1948. From the start of their marriage, the couple spent half the year on the Italian island of Ischia, and by the mid-1950s they lived there permanently.Walton's last work of the 1940s was his music for Olivier's film of Hamlet (1948). After that, he focused his attentions on his opera Troilus and Cressida. On the advice of the BBC, he invited Christopher Hassall to write the libretto. This did not help Walton's relations with the Sitwells, each of whom thought he or she should have been asked to be his librettist. Work continued slowly over the next few years, with many breaks while Walton turned to other things. In 1950 he and Heifetz recorded the Violin Concerto for EMI. In 1951 Walton was knighted. In the same year, he prepared an authorised version of Façade, which had undergone many revisions since its premiere. In 1953, following the accession of Elizabeth II he was again called on to write a coronation march, Orb and Sceptre; he was also commissioned to write a choral setting of the Te Deum for the occasion.Troilus and Cressida was presented at Covent Garden on 3 December 1954. Its preparation was dogged by misfortunes. Olivier, originally scheduled to direct it, backed out, as did Henry...
Answer:
Walton