Hard to say, but I suspect it wouldn't have been much different. Blair had become very unpopular towards the end, but Brown was too by the time of the election, so I doubt there are that many people who voted Labour with Brown who wouldn't have done with Blair (and they'd probably have mostly voted Lib Dem or other rather than Tory).
I imagine moving 1-3% of the votes from Labour to Lib Dem would have some interesting affects for the Tories.
Yeah, a majority...
Not necessarily, it depends on where the votes move, as swings are never uniform. If the swing was uniform, though, the result with a 3% swing from Labour to Lib Dem would look like this:
Conservative: 310
Labour: 238
Liberal Democrat: 73
Other: 28:
Figures calculated from here:
http://ukpollingreport.co.uk/swingometer-mapEDIT: Corrected spelling of Labour.
