Macmillan certainly hinted at it though he was 82 years old in 1976. I think if there were a national disaster - like the collapse of the sterling balances, a huge oil crisis or industrial strife or a military crisis, a National Government would be plausible (there was a lot of Second World War nostalgia among the top brass of the Civil Service and politicians during the crises of the 1970s, Heath's Principal Private Secretary and the Head of the No. 10 Policy Unit predicting the return of food rationing for example...). Though I doubt that Macmillan would lead such a government - a more active politician like Heath, Healey, Jenkins, Maulding, Whitelaw would be floated. Several leading Conservatives of the late 1970s like Maudling floated, albeit flippantly, the idea of supporting a National Government under Denis Healey, although he was probably too tribal to accept if things came down to the wire.