OK. Fixed it. I have to admit to a certain amount of redness in the facial area.

I didn't go far enough in the testing of the received email. The displayed heading in my Windows Mail Inbox of was of the form "replytoname )" and in my innocence I believed that email address to be the Reply To address. I never actually tried to do a reply to it. When I did that today, more out of frustration than serious testing, lo and behold it provided the correct email address in the To field. i.e. from my code above.

Case closed.