Well I personally have never believed Miliband will let David Cameron stay in number 10, if by courting the SNP he will bring the Labour Party to power. He's surely not that dumb.
His somewhat idle promise is surely being done in an effort to placate Labour voters on both sides of the border...
Jeeeezus ! Please don't tell us you believe anything politicians promise! And if any elections had been voided through idle promises we'd never have a government..... they will say anything to get power, all of them. Some worse than others,true.