I have got to the point where I can do most of what I need, provided I am on the attachment page (in a child theme). For a attachment page of a given post ID say 201 If I call wp_get_attachment_url(201) I’ll get the correct URL, e.g. the URL to the high res image like http://127.0.0.1/hiresimages/windsurfing/Baja-2013-8142.jpg.