It would be nice to think that one in five would not consider voting BNP.
YouGov has in the region of 250,000 on their panel, so to actually shift the result of an opinion poll by 1 point would require about 2,500 people. More to the point, it would require 2,500 joining in a way that several full time professional panel management people did not notice. A sudden surge of 2500 new members, who all joined through the open website (most people are recruited pro-actively) and who all answered questions in a particular way would be pretty bloody obvious.
There’s more, plus a further bit of commentary by Peter Kellner.