Some further analysis from my side.
The page https://www.citrusandsea.com/booking-confirmation/ contains not one, but two nested forms.
<form class="mphb_sc_checkout-form" enctype="" method="POST" action="https://www.citrusandsea.com/booking-confirmation/?step=booking">
...
<form method="GET" class="mphb_sc_search-form" action="https://www.citrusandsea.com/search-results/">
</form>
...T&C acceptance checkbox...
<p class="mphb_sc_checkout-submit-wrapper">
<input type="submit" class="button" value="Book Now"/>
</p>
</form>
Although, this may be a red herring.
