Do you still live in Huntingdonshire? I've just done a quick wiki on the constiuency. Yes, it's 'true blue' - but at the last election the Conservative candidate won with 26,516 votes, the next closest was the Lib Dem candidate with 15,695 votes - but around 24,000 people on the electoral roll did not vote! If those 24,000 people had turned out and voted LibDem, or Labour (who got around 5,000 votes), the result would be different.
The problem in seats where the same party have been returned over and over is people get like you are now, they don't see that it's perfectly possible for someone else to win. Little effort is put into winning these people who don't bother to vote, and lots of people who would vote for the other 2 main parties don't bother because what's the point, the party who always wins will win - and then they do.
Some people vote in a tribal way, they have voted for a particular party their whole life, their family all vote that way, that party is who they feel should represent them, increasingly that's not the case anymore, the old ties to any party are reducing and there's a lot to be played for.
Of course, in a very rich consituency, you'll find that the Tory policies are most likely to be the ones that will best serve the consituents, in a poorer consituency Labour's policies are more likely to be the ones that best serve the consituents, so even if people do judge freshly each time, then the people living there are likely to pick the same party again, that doesn't mean they have been 'sheep-like' in voting for who they've always voted for, and could be swayed by another party if the other party was the in their best interest.