I have created a custom view that has the items grouped by
conversation. My question is; is there a way to filter the view so
that it will show only those conversations with a single mail item.

To give you a better understanding of what I am after. We have a
public folder where questions can be submitted. We are looking for a
simply way to identify those questions that have never been responded
to. Hence, viewing by conversation and showing only those with a
single mail item will be enough to identify those which need to be
addressed.