I've inserted the form onto the web app item detail page using the following code which references a .tpl file where the form is located:
<div id="booking-form">{module_booking,i,{tag_booking id}
template="/ModuleTemplates/Events/{tag_payment type}.tpl"}</div>
The form has the auto-responder turned on but as mentioned, when I test the form it doesn't send the contents of the auto-responder from the form, it sends something different as mentioned in the original post.
I actually saw a tutorial on this at the following site:
Handle Paid and Unpaid Event Registrations with Business Catalyst
It didn't mention the auto-responder though.
Maybe because of the way I've set it up, perhaps it's not possible to send the auto-responder from the original form, do you think?