It is hard to be more specific about this date, although there are some references to it among Britten's correspondence. The composer wrote to Elizabeth Mayer on 30 Sep 1942, remarking that his 'Sonata for orchestra limps along between radio jobs'. However, one of the sources for the work is on paper Britten acquired in the US and is among sources for works written in 1941. There is a further letter mentioning the work from Ralph Hawkes (Britten's publisher) on 28 Jun 1943 in which he suggests that the work could be complete before November of that year. It is, then, possible Britten had planned to continue working on it during 1943. See Donald Mitchell and Philip Reed, eds., Letters from a life: selected letters and diaries of Benjamin Britten (London: Faber, 1998), vol 2: 1939-1945, pp. 1042-3 and 1089.