You Gov has it at Boris Johnson 44% and Ken Livingstone 36%. If no one gets 50% second preferences are allocated and on second preferences from the Lib Dem candidate mainly it becomes 53% to 47% for Boris.
Mori however has first preferences Boris 38%, Ken 41% and second preferences Ken 52% Boris 48%.
A lot of interest in who will be right. You Gov does Internet polling and Mori phone polling. You Gov has a pretty good accuracy history but that is no guarantee. What they do tend to agree on though is the higher the turnout, the better it is for Ken Livingstone.
The Livingstone campaign has actually complained to the UK Market Research Society about You Gov. This shows how seriously people take the expectations game.