Well famously Labour DID win the 1951 election, just by votes not seats, indeed they got more votes than they did in their 1945 landslide, however such is the arcane nature of the Westminster system.
I agree, probably easier to give Labour a stronger majority in 1950, say a scandal or two doesn't hit the Attlee Government that did OTL, say Chancellor Dalton doesn't cock up and hand out Budget leaks to a journalist (and doesn't let that social-sadist creep Cripps into the Exchequer). If Labour can hold strong until 1954/1955 when the economy began to go well and Gaitskell will probably be Chancellor, there's a chance they might win a third term, after all Labour can quite rightly at least in PR terms can claim the success of Britain's post-war rebuilding.
The reason Chruchill lasted so long as Tory leader was because he refused to stand down and his (Shadow)Cabinet knew any coup would be viciously attacked by the rank and file of the Party. By 1955 we'll be seen as increasingly out of touch and quite ill, either Eden will finally have the balls to oust him (which might hurt the Tory reputation close to an election) or Churchill's health will do the dirty work for him.