YouGov have been the most reliable pollsters over the last few years, so it's interesting to see that they are being accussed of having a pro Labour bias. During the London mayoral election they were accused of having a pro Tory bias but they were vindicated by the actual result. Whether they are right this time remains to be seen but their track record indicates that they aren't motivated by political bias.
In any case I don't see how boosting Labour's apparent level of support this close to an election would benefit anyone except the Conservatives. If Labour are thought to be in with a real chance of clinging on then it will do wonders for the Conservative turn out.