As can be seen from the charts above the biggest problem the pollsters had was with LAB which had a huge range in the final polls. Partly, I suspect, this was that Labour supporters had not finalised their decision to vote tactically when they were being questioned.
It is well know that a significant proportion of voters really don’t make their minds up until the last moment. This is why Ipsos MORI generally try to ensure that its final poll before an election involves fieldwork carrying on until latish on the Wednesday evening.
Matt Singh of NCP explained in last week’s PB/Polling Matters Podcast how his final numbers had been impacted by a large proportion of his sample doing the online questionnaire on the the Saturday.