What is Labour's bedrock level of support?

I have previously blogged that I thought the low 20s was the bedrock of Labour support and that I couldn't see it dropping below that. Political Betting today report that the latest YouGov poll for The Sun shows these voting intentions:
Conservative 41% (down 2)
Labour 22% (down 5)
Lib Dems 19% (up 1)
Others 18% (change not mentioned but I assume must be up 6)


I know that close proximity to elections that are not a General Election boosts minor party votes but this looks bigger than normal. I am sure that UKIP are gaining "protest" votes but on these figures it looks as though more voters are deserting Labour than the Conservatives. Maybe they are moving to The Greens, that is certainly the idea that the BBC are pushing this morning but I think that more are moving to the BNP and that could make the election results very interesting and maybe mark the end of Labour's love affair with Gordon Brown... Alan Johnston Prime Minister by the end of the summer?