Jackdaw,
It is worth remembering that in the 1971 Commons vote on joining the EU, 39 Tory rebels voted not to join, and Heath had to rely Labour MPs to get our EEC membership approved by Parliament.
The left of the Labour Party has always been anti-Europe. Thus, when ypu say 1975 Labour was pro-Europe, it wasn't. There was a free vote in the referendum, and many Labour MPs were in the OUT camp, most notably Tony Benn.