I actually happen to think the SDP help, rather than hinder, the possibility of a left wing Labour government. There are plenty of Tory people who would not vote for Benn or Foot, but would vote for them. Running the 1983 figures through electoral calculus, Labour emerging as the largest party in a hung parliament even when finishing third in the popular vote is not at unlikely if the Alliance wins the most votes. Of course, you'd probably want a Labour majority for this scenario, but nevertheless, a large but even more unfocused Alliance vote (perhaps in an election that takes place within a year after their founding) could see Labour in with a majority as long as they could remain substantially ahead of the Tories. You could even have the Alliance edge the popular vote, thereby resulting in more people questioning the legitimacy of a Labour government, and dividing the opposition at the same time with about 140 MPs each, giving Labour a better chance at winning a second term.You'd also need to do away with the SDP split, which is perhaps the hardest aspect to achieve: the further left Labour goes, the more likely a split occurs and the more successful a split is likely to be. I don't see any way that a split doesn't happen so the challenge then becomes how to render it ineffective? Perhaps a series of (orchestrated?) scandals?
Another consequence would be that it would get rid of a lot of troublesome right wingers, which makes it easier for a Labour government to be unabashedly left wing.