Through a process of elimination, I've narrowed this down to two issues. The "undefined method..." error seems to be related purely with the form validation functionality. If I leave any of the required fields blank when submitting the form, the error appears when the page reloads. For now I've just removed any requirements, but I'd love to get to the bottom of that one.
Additionally, in my "mailer" page part, I had: from_field: sa...@mydomain.com and for a hard-coded email address (as opposed to an email address extracted from the form fields submitted by a site visitor) this should instead be: from: sa...@mydomain.com After making these changes the form now submits, the redirect occurs, and an email gets sent. Thanks for your help, JT