I think Norwich South shows what went wrong with Labour over their three terms of office. This first graph is the majority that Labour had from 1997 in a seat that has been a Labour seat for a decade before:
A huge 14,000 majority in 1997 has shrunk so much that the Labour candidate lost at the 2010 election by 310 votes. Why was this the case? Well, let's ignore for a moment the candidate who achieved the 14k majority and lost it last year, and instead look at how people have voted in that constituency. In this graph I have plotted the votes for the three main parties, and the Greens:
Now look at the British Attitudes Survey from 2009
This shows a similar convergence between Labour and the Conservatives. (The Lib Dem bounce in the year before the 2010 is not recorded here.) There is a rise in those people who regard themselves as having no political affiliation, or don't know, and 'other' also increases. In Norwich South the Greens have benefited from the rise in 'other' and possibly from those people with no affiliation choosing the Greens as a protest vote.
Of course, it could be the Labour candidate who inherited a constituency with 6k majority at a time when there was a national mood for a Labour landslide and then, after gaining a 14k majority in 1997, allowed it to slide by his behaviour in Parliament both as a minister and as a backbencher with a grudge. The losing Labour candidate was, of course, Charles Clarke.