Problem Identified:
You are not using a standard WordPress theme and intentionally not using get_header(), get_footer(). You are also avoiding wp_head() and wp_footer() so all plugins and themes Scripts and Styles aren’t loaded. My plugin requires JavaScript files for sure.
Details are here:
@phildotpot please try this plugin with a standard WordPress theme using WooCommerce checkout. What i see, your checkout page isn’t working as expected. Your files are located on root, even css etc. Checkout page is maybe a custom developed page which is submitting to WooCommerce engine? Not sure, kindly use my plugin with a proper theme and WooCommerce combination. I am available for bug fixing and troubleshooting but custom made incomplete themes and scripts aren’t encouraged as each case will bring a matrix of complexities.
Have a look here, this is WordPress twentyseventeen theme with WooCommerce. Try add to cart and process checkout, you will see my plugin working as expected.
https://demo.gpthemes.com/product/bread/