Thanks for your reply Jason
my confusion was due to the bootstrap example elements, using default input elements will work fine - see example below
<button class="btn btn-default" type="submit">Button</button> doesn't show in trigger field
<input class="btn btn-default" type="submit" value="Submit"> shows in trigger field
Why I tried the button element was due to the following recommendation on http://getbootstrap.com/css/#buttons
" Cross-browser rendering
As a best practice, we highly recommend using the <button> element whenever possible to ensure matching cross-browser rendering.
Among other things, there's a Firefox bug that prevents us from setting the line-height of <input>-based buttons, causing them to not exactly match the height of other buttons on Firefox."