Okay, so this thing where whatever the polling industry does, it seems, they never quite get Labour's voteshare right:
Is it possible there's a sort of Heisenberg's Uncertainty Principle thing? The very act of measuring Labour's vote share moves the polls?
If Labour are doing really well (1997, 2024), fewer people vote Labour, because a lot of them are doing it to get the Tories out. If Labour are doing badly (2010, 2017), they get sympathy votes drifting back.
I think so, yes. If the party's comfortably winning, it's easy to indulge your preference for the Tooting Popular Front as a protest against Labour policy on bin collections. If it's way behind, the prospect of unfettered Tory power probably focuses a few minds.