The official statement from the British Polling Council says although “… the final polls were not ideal, the BPC does not feel there is a need for another formal inquiry”:
“The polls proved very good at predicting the Conservative vote, with an average of the final polls being only 0.2% higher than the actual result… They did, however, prove much less good at predicting the Labour vote, with the pollsters’ average being 5.2% below Labour’s actual share. This is only the second election since 1987 when the pollsters have underestimated the Labour share. The average poll figure for the Liberal Democrats was very close, being only 0.3% different from the result.”
Guido has calculated the above accuracy league from the BPC statement. The official final polls as per the BPC (and the BBC) show that the pollsters were on average out by 1.97%. Which could easily be a decisive margin in a general election…
Data here spreadsheet monkeys.