How much of a majority were the conservatives expected to have before the voting began? (NOT the exit poll that began after voting did)
The betting was 340-350
The polls varied a great deal but some of the last pre-election ones were suggesting a majority of low single digits to perhaps twenty. There were a lot of fears that if the polls were off by even a bit, there could be a hung parliament that might allow Corbyn to form a coalition government with the Liberal Democrats and the SNP.