Simple JQuery Script for woocommerce not working

This is quite a tricky one to debug as the problem occurs in the context of an entire page and I struggle to reproduce it in isolation. I will try and post as much relevant code as possible here though.

Essentially I have a dev WooCommerce WordPress store for which I have built a custom theme. On the checkout page there is the standard choice of payment options marked up as follows:

<ul class="payment_methods methods">
    <li class="payment_method_bacs">
        <div class="radio">
            <label for="payment_method_bacs">
                <input id="payment_method_bacs" type="radio" class="input-radio" name="payment_method" value="bacs" checked='checked' data-order_button_text="" />Direct Bank Transfer</label>
        </div>
        <div class="panel panel-default payment_box payment_method_bacs">
            <div class="panel-body">
                <p>Make your payment directly into our bank account. Please use your Order ID as the payment reference. Your order won&#8217;t be shipped until the funds have cleared in our account.</p>
            </div>
        </div>
    </li>
    <li class="payment_method_cheque">
        <div class="radio">
            <label for="payment_method_cheque">
                <input id="payment_method_cheque" type="radio" class="input-radio" name="payment_method" value="cheque" data-order_button_text="" />Cheque Payment</label>
        </div>
        <div class="panel panel-default payment_box payment_method_cheque" style="display:none;">
            <div class="panel-body">
                <p>Please send your cheque to Store Name, Store Street, Store Town, Store State / County, Store Postcode.</p>
            </div>
        </div>
    </li>
</ul>

The idea is that when a payment option radio button is selected the corresponding payment_box is shown. Upon referring to the plugin assets there is a jQuery snippet that achieves this:

payment_method_selected: function( e ) {
            if ( $( '.payment_methods input.input-radio' ).length > 1 ) {
                var target_payment_box = $( 'div.payment_box.' + $( this ).attr( 'ID' ) );

                if ( $( this ).is( ':checked' ) && ! target_payment_box.is( ':visible' ) ) {
                    $( 'div.payment_box' ).filter( ':visible' ).slideUp( 250 );

                    if ( $( this ).is( ':checked' ) ) {
                        $( 'div.payment_box.' + $( this ).attr( 'ID' ) ).slideDown( 250 );
                    }
                }
            } else {
                $( 'div.payment_box' ).show();
            } //etc...

This script is included in my site scripts by WooCommerce in the normal way and there is nothing in the console indicating a problem but for some reason when a payment method radio is selected its corresponding box does not show.

I think the only way to demonstrate is to link to the site in question: http://ift.tt/1DQlDsM. Add a product to cart and visit http://ift.tt/1Hsn6TO.

If anyone can help me debug this I'll be enternally grateful!