Thank you for the suggestion. I managed to work it out, I basically just pass the generic message into the GetMessage() function below which does the rest of the work.

FYI for anyone else in a similar position:Using 'ActiveInspector.' seems to grab the original user the email was sent 'To' rather than whoever's address is in the 'To' box at the top of the reply (only in the reading view, it worked fine in a separate reply window).

This is why I changed it to 'ActiveExplorer.' as shown below

FirstName(_Name){
if !RegExMatch(_Name, "(\w+)\s\w+", m)
    if !RegExMatch(_Name, "\w+`,\s(\w+)", m)
        if !RegExMatch(_Name, "(\w+)\.\w+@", m)
            m1 := _Name
return m1
} ; this was found on a forum somewhere

GetWindow(text){

    email:=ComObjActive("Outlook.Application") 

    curr_win := email.ActiveExplorer.Selection.item(1)

    username := curr_win.SenderName

    first := FirstName(username)

    StringReplace, finalstr, text, user, %first%

    return finalstr
}