Briefly, the two charts below show firstly mentions of the party leaders without collocation with mentions of the other leaders, and secondly mentions of the party leaders without this restriction (i.e. without requiring that the name appear in isolation).
Gordon Brown's microphone gaffe is clearly visible (the second blue peak). The first graph can be thought of as a description of each candidate's recognized identity. Notice how Brown is isolated in the first graph around the unfortunate microphone story.