Word isn't finding hidden text

I'm trying to find a workaround for the issue I'm having with duplicate entries in my document's index.

My plan was to laboriously go through all of the fields marking index entries, then replace every occurrence with a standard entry using search & replace. i.e. search for XE "Castles" hiddent text and replace every occurrence with a standard character format.

It doesn't work, as shown in the accompanying screenshot. Word tells me that there are no results, even though I can see one (highlit) on the page.

Thanks in advance,

Steve