Whatever the rights and wrongs of Corbyn's position, Harold Wilson was *not* neutral in the 1975 referendum. Here he is in Cardiff, campaigning to "Keep Britain in Europe". 
Wilson never pretended to be "dappy" about Europe - as he once put it, "I don't stand on the South Coast, look towards the Continent, and say 'there's a new Jerusalem'" - but his support for the Yes campaign in 1975 was one of its strongest assets. From my book, "Yes to Europe":
The key task for the Yes campaign in 1975 was to give sceptical Labour voters, who saw EEC membership as a Tory policy, a reason to change their minds. Wilson was better placed to do that than a Euro-enthusiast like Roy Jenkins. So the Yes vote was very much Wilson's achievement.
It also allowed Wilson to marginalise the Bennite Left in the party, if only temporarily. This was how his advisor, Bernard Donoughue, described the strategy:
