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Your primary div ID selector is missing the alignleft class on your post page: http://tayl0r.co.uk/archives/advent-calendar/1-december-2014.

Where your main page http://tayl0r.co.uk/advent-calendar includes it which adds the following to the primary div:

.alignleft {
display: inline;
float: left;
}

This forces the two columns to play nice together.

You may have to edit your theme to add (if there isn’t a theme setting you can tweak) or add the additional CSS (mainly the float: left;) to the primary div ID selector or the content-area class (but you’ll need to test to make sure it doesn’t impact other pages if you do this).