Slightly off-topic, and I hope not overly-cynical but
"unless there's an election every 5 years, how can we hold our MP's to account?"
I think that "holding to account" is an over-used term in regard to politics.
In a 2-party democracy (e.g. UK, US) I don't think that elections can really be a way for the electorate to hold politicians to account - particularly on single issues - since the choice is binary (as a vote for a minor party is likely to be marginally dilutive at best). You are voting on a manifesto - a package - regardless of whether you agree with all of its contents.
You might see protest votes in local council or by-elections but not in a General Election.
Given the binary choice, I would suggest that most people won't switch allegiance between the side that they have settled on.
Therefore if you "hold MPs to account" at the General Election (assuming they are the ones you voted in last time) then you will get the lot you didn't want and have never wanted.
I don't think PR helps either, since the pool of parties (and hence manifestos) is still pretty small.
Do we need to discard political parties, but vote for policies and have them delivered by a neutral Civil Service? But then who decides on the policies, if they are consistent/deliverable/mutually-exclusive and who is actually in charge?