No, I had already tried that. It places the subentry at the top of the list of subentries, not in its correct alphabetical order. Using the example below, I assume you meant (I am including text before and after):

The \index[Plenary+periti]{Plenary Council+{\it periti} (experts)} group comprises 10 members.

There are nine subentries altogether, and quite a number under 'p', like \index{Plenary Council+preparations for}. The italicised periti should come just before the latter (preparations for) but, using the double key above it comes first in the list. The same happens with the subentry surrounded by single quotes. It too comes first in the list if I use a double key. So clearly it is the fact that there is formatting ({\it something}) or quotes ({‘word’}) in a subentry that causes the problem, and I am wondering how to overcome this.

Julian

2. \index{Plenary Council+{\it periti} (experts)}: in this case it is the italicised /periti/ that appears out of place, after the letter 'i' rather than after 'p'. Again I tried putting various keys but this did not help.

I guess my confusion is this: I assumed that the [key] establishes the literal string which determines sort order. That seems to be the case for a main entry. How do I get it to work for a subentry?

just provide an extra key: