MORI monthly political monitor
Ipsos MORI's monthly political monitor has been published and has figures of CON 40%(+2), LAB 24%(+3), LDEM 18%(-1). I don't have the figures for "others", but looking at the changes from their last poll we can assume that, like all the other recent polls, they are in decline.
In this case, it's Labour who have gained the most, but together with the other recent polls that have shown the Lib Dems or Conservatives with the largest increase I think we can reasonably conclude that the decline in support for minor parties has not disproportionately benefited any of the big three.