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Doesn't work anymore, Trainline is checking the box multiple times during page load...

Here is my fix:

(new MutationObserver(check)).observe(document, {childList: true, subtree: true});

function check(changes, observer) {
    let c;
    if ((c = document.getElementById('bookingPromo')) && c.checked) {
        c.click();
        c.parentNode.parentNode.parentNode.style.visibility = 'hidden';
    }
}