An idea for political reform: a party's political manifesto should be a legally binding contract

One of the readers commenting on this story makes an interesting point about accountability in politics.

"You will only have true democracy when you have the "right of recall". A party's political manifesto should be a legally binding contract. They are elected on the promise of doing A, B and C and instead they do X, Y and Z. That should be illegal. They should be forced to call another election if they cannot deliver on their manifesto."

Sounds great in theory, but would it work in practice?

Surely a massive challenge would be defining exactly what counts as having "delivered" on an election promise?  If a party promises to provide £10m in funding for a project but ends up only finding £9.5m, is that reason enough to call an election?

Wouldn't the temptation be to make your manifesto as vague as possible - therefore actually reducing accountability?

Anyone?