Here are the results of an opinion poll for the forthcoming Norwich North by-election:
CON 34%(+1), LAB 30%(-15), LDEM 15%(-1), Green 14%(+11)
If accurate, it looks like the Conservatives will gain this seat from Labour.
But look at what has happened to the vote. Labour's support has dropped massively, and has mainly gone to the Greens. Now analysing political parties simply in terms of left-right is a crude measure, but nonetheless it is obvious from a quick glance at their policies that Greens are to the left of Labour. This poll represents a significant leftwards shift in attitudes in Norwich North.
So a shift to the left results in a gain for the right. Yet another flaw with first-past-the-post.