Thanks for your help Robert.

For the benefit of anyone with a similar query:
^f doesn't work in the 'replace with' box, and to make an alteration while keeping the footnote itself unaltered you'll need the wildcard code for ^f, which is ^2.

So to introduce a space between the text and the superscripted footnote number I used:

In the Find box: (^2)
In the Replace box: space\1
(with wildcards selected)