The results of publicised opinion polling for individual constituencies are detailed in this article.
However, most opinion polling covers Great Britain, where the results are published in this article here.
Under the first-past-the-post electoral system true marginal seats are, by definition, decisive as to the election outcome.
[2] Note: Michael Ashcroft initially published incorrect results for his 18–23 Nov 2014 poll,[3] erroneously showing only a small Labour lead.
When asked to think only about the political parties [the methodology used by the Ashcroft polls] the figures showed a small Labour lead.