Poll Accuracy
The graph below was not done by me, but e-mailed to me by a friend, so I can’t vouch 100% for it, but I took a quick look at the data and it looks right to me.
The graph compares the average of the polls published just prior to a general election with the actual result. It shows that the polls on average have been consistently close to the actual result for the last three elections. Not perfect, but generally within 1.5%.
Past performance is no guarantee of future results, but it is a fairly good indicator.