Thanks to those who had a look at this. It is now fixed, after quite a bit of trial-and-error (manual adjustment of the margins and columns). The problem is that I would have to go through the same process again (which is quite fiddly) if I did Update Field.

In summary, the Insert Bibliography formatting only seems to work properly when there are less than 10 references (i.e. when the reference number format is [x]). I have over a hundred, and it won't cope with [xxx], or even [xx].

Regards, Phil