Traditionally Burton was a Tory seat; not hugely safe but not a marginal either.
And that's despite its demographics having a slight working class lean (it didn't even fall in 1966). The reason for that o/c is that the main industry in Burton is beer.
Btw, before '45 Burton was one of the safest Tory seats in the country (the Tories usually ran unopposed and when they didn't they won by absurd margins. Hard not to get the impression that the brewers put a certain amount of pressure on their workforce...) and its loss to Labour that year must have been huge.