What do Loughborough, Northampton, Watford and Dartford have in common? To most people, nothing really, and they would be right. To the psephologist, however, it is obvious – they are all so-called “bellwether” parliamentary constituencies. Put simply, the results in these towns will reflect the national outcome of a general election and there is no bellwether quite like Dartford. Since 1964, the north Kent town has returned an MP from the winning party, whereas the others mentioned have been accurate for only 50 years. Election watchers look to these seats for changing trends in voting habits…