Liberal Democrats and first past the post

So what if Labour is pushed into a poor third place and the Liberal Democrats come a close second to the Tories?
How will proportional representation then look to the Liberal Democrats? If as is possible, Labour self-implodes it is the Liberal Democrats who stand to gain the most at the next general election. So much so that under a first past the post system the Liberals would then have the ability to challenge the Tories for outright power!
On the other hand proportional representation at the next election would leave the door open for the Tories to do deals with other parties, leaving the Liberals stuck on the back benches.
If there is a hung Parliament, the Liberal Democrats might like to think twice about having proportional representation for MPs at the top of their negotiating list.