Update:
I've managed to get the ASP content into (and save back out of) the editor by using the TextStream object. The only remaining problem is an included file that sits inside some HTML comments, thus:
<!--#INCLUDE file = "headers.asp"-->
Which the editor seems to ignore, even with
editor.AllowEditServerSideCode = true
Is there a way to get the editor to keep the include statement? It would be a drag to have to incorporate it programatically although this is what I shall do as a short-term fix.
Steve