Anthony Wells is arguing about what percentage of don't knows in the polls for the Crewe & Nantwich byelection are actually Labour voters who feel embarrassed to say so - analogous to the "shy Tories" of the 1990s. I would think the number is considerable.
During my candidacy in Egham Hythe, I knocked on and got an answer from around 100 doors. In the event, there were roughly 600 Conservative votes, 300 Labour, and 200 Liberals counted. My own canvassing numbers logged 22 Liberal, 28 Tory, 1 BNP, 1 UKIP, 31 Don't Know among those who said they would vote...and 4 Labour. Yes - four. 5.19% of the total answers, as against 30% of the vote. Redoing the sums, assuming the same pattern for the non-answerers, predicted 51% Tory, 40% Liberal, 7.3% Labour (note this is a two-councillor ward, so votes for candidates must be divided by 2); the event was more like 60% Tory, 30% Liberal, 20% Labour, 10% nutters and spoilt ballots.
Conclusion: There are a *lot* of quiet Labour voters out there.