Sorry, Guest - she would normally ask the leader of the biggest party to form a government because its leader would command the support of a majority of MPs, but if no party had a majority, she would take advice (from privy councillors?) about who to send for. It could be the leader of the second largest party who'd already done a deal (or thought he could) with a lesser party. In the nineteenth century, after a governing party had been defeated in an election, it continued in office until it was defeated in a Commons' vote. Constitutionally,this is still the case. Remember Heath in February 1974? Although defeated by Wilson, he didn't concede for a day becauyse he thought he might be able to to deal with Thorpe's Liberals.