Hey Danny,
Thanks for the reply! I did not paste any (short)code into the page or frame/element manually – the restaurant reservations plugin created a menu in the plugins section of wordpress from which you can select the page to which you want the form added. So I guess the short answer is I used the widget.
I get the feeling it would work if I could do it the other way (ie by using a link or shortcode to embed the form manually into the pagebuilder row?)
Matt.