Turnout tends to be low if people think that there is going to be an easy victory.
If Labour support evaporates in the next few weeks, as it did during the petrol protests last autumn, the prospect of a close race may galvanise voters. The problem will be compounded in safe Labour seats. At least in 1997 there was the excitement of at last getting rid of the Tories. But that motive no longer holds. Received wisdom suggests low polls are worse news for Labour than for the Tories.