Through reading other post I found that the view responsible for this was Parts_Common_Body_Summary. So I copied this from the core / common folder of orchard and copied it across to my themes view folder, before renaming it to Parts_Blog_Summary
I then set-up a rule for this in Placement.info as follows:
<Match ContentType="BlogPost">
<Match DisplayType="Summary">
<Place Parts_Common_Body_Summary="Content:after;Alternate=Parts_Blog_Summary"/>
</Match>
</Match>
This just left me the task of altering the string length in the new alternate view:
var body = new HtmlString(Html.Excerpt(bodyHtml, 350).ToString().Replace(Environment.NewLine, "</p>" + Environment.NewLine + "<p>"));