In the bad old days of Tory govt, we got to a situation in which opinion polls significantly underestimated the Tory vote. This was said to be because some people who voted Tory were shy of telling pollsters that they would do so. The idea, which may well have been true, was that voting Tory was seen as nasty or selfish, so some people kept quiet about it.
Once the polling organisations were forced to admit that their polling was inaccurate - following the 1992 election, IIRC, when the polls got it surprisingly wrong - they started adjusting their data to take into account the strange phenomenon of the shy Tory voters.
Does anyone know (i) whether there is any evidence of the shy Tory phenomenon now and (ii) whether pollsters are still making adjustments to the data so as to avoid underestimating the Tory vote?