Is there such a thing as a safe parliamentary seat?
From the 2009 EU Elections.
45 million eligible voters
15 million actually voted
30 million did not vote.
In the EU election the Conservatives gained the highest percentage of votes ~27% which is just over 4 million votes.
It doesn't take a math genius to work out IF all 30 million non voters decided to vote as a group, whoever they voted for would win by the biggest landslide ever. And this would be true in safe seats as well.
So although this is not going to happen, the principle is there.
It's basic maths, not saying it will ever happen, but a more realistic example is a place like Boston & Skegness had a 58.8% turnout out of 71,212 eligible voters.
This means over 29,000 people in that constituency didn't vote.
The Conservatives won with 19,000 votes (Labour got 6,000 fewer votes) so again those 29,000 people who think their vote is a waste of time COULD have determined who won. Would have taken 7,000 to put Labour in power, 16,000 for UKIP or Liberal Democrats, 18,000 for the BNP and 19,000 for the Green party.
So any party could have taken power if enough people who think voting is a waste of time got of their collective arses and bothered to vote.
Do the maths for yourself:
Turnout figures at UK General Election 2005 Voter Turnout