Let's not forget the hilarious intra-hard-left sectarianism. I believe it is SLP policy to run in constituencies where another hard left candidate is running. So yeah... (Is this more evidence of the "Scargill was MI6" theory? It is quite suggestive)
But more seriously, hasn't that been the case since... well, the 1950s at least? While the hard left certainly had a lot of influence in the unions and sections of the labour party, independent leftists did not... err, fare so well in elections without those structures (since the decline of CPGB anyway). Or am I missing a part of the story?