That said, the actual 'effective Labour majority' immediately after the General Election is slightly complicated. The General Election was held on 5 July 1945, but the polls in some constituencies were delayed until 12 July with one constituency delayed until 19 July. Moreover because of the war it took time to get the huge number of overseas votes from British forces personel stationed around the globe, counted and then all these results communicated back to Britain. Accordingly the final result wasn't declared until 26 July.
But in the meantime two Conservative MPs, duly elected by their constituencies, had died: Sir Edward Campbell died on 17 July and Leslie Pym died on 18 July, both before the final result was announced. By-elections for their replacements were not held until August. Similarly the sitting MP for Hull Central, Walter Windsor (a Labour MP), died during the election campaign on 29 June, and as a mark of respect his seat was not contested during the General Election; the by-election for it was again held in August when it returned another Labour MP, Mark Hewitson.
Nevertheless, however you tally it up, the final result was an unexpected landslide victory for Clement Attlee's Labour Party over Winston Churchill's Conservatives.