Is Labour really on 19%?

One thing that few people seem to have noticed in the mainstream media is the correlation of the Labour Party vote between the European Elections and the recent Norwich North by-election. Some of you may remember that I said of the Labour Party European result (15.9%) that it would be almost impossible to believe that Labour would do anything like as badly in a Westminster contest.
However, the result from Norwich North actually seems to confirm that between a swing to the Tories and the Labour vote not coming out the effective Labour vote may actually be somewhere around the 20% mark. While polling still has Labour on about 25%, I am starting to wonder whether there is a large 'stay at home' percentage in those numbers, which would reduce Labour's support to something approaching 20% nationwide.
Now, I can say these things safely because, due to the amount of time between now and a general election (and the amount of stuff that can happen in the intervening period) it is unlikely that my hypothesis will be tested. But, we can give it a trial run in the Glasgow North-West by-election (whenever Labour get around to holding it). If there is a corresponding dive in the Labour vote to less than 20% (although it is such a safe seat that even a higher percentage of the vote could be indicative of a dangerous nationwide picture), then we could be seeing three things on election day.
Firstly, if Labour is really getting only 20% of the effective vote, the Liberal Democrats will almost certainly take second place in the popular vote (and will possibly be threatening a few Labour heartland seats).
Secondly, the Tories will win an absolutely massive majority (about 150, although this depends on the Tory and Lib Dem shares of the vote as much as Labour's).
Thirdly, we could see a serious advance by the Nationalist parties in both Wales and Scotland. Particularly in Scotland we could potentially see same very safe seats falling to the SNP as Scots show their disgust with the government not by voting Lib Dem or Tory, but for the Nationalists. We must remember that the SNP have been running the devolved government in Holyrood for the past two years. Many Scots see them a realistic voting alternative for political, rather than policy reasons (meaning to say, you don't have to be pro-independence to vote SNP any more). Glasgow North-West will be a fascinating contest in this regard...