"What politicians say before an election is designed to get people to vote for them. It need not bear much resemblance to what they do once the votes have been collected."

That is perfectly true and of course another scenario is that a Tory party with the biggest number of seats but no overall majority may decide not to go into a permanent coalition with the Lib Dems (who would almost certainly demand PR in any such coalition) but may decide instead to try and work on with a minority government which means they may need to work out deals on specific issues only. It is not so far fetched as there is a precedence with the existing SNP administration in Edinburgh who have no majority. They've managed to get most of their bills through but have had to abandon certain measures where they have no support from elsewhere.   

Another scenario with this is that if they bring forward their English MPS for English matters idea - then it is perfectly possible that they could have a real working majority on many of the proposed Westminster bills anyway once Scots, Irish and Welsh based MPS are discounted.