Ignore this question, it's sorted.
Just for everyone's reference, it worked when I added only the gtag part of the event snippet as non-sequential JS
gtag('event', 'conversion', {
'send_to': 'AW-XXXXXXX/abcdefghijk',
});
whereas the entire event snippet is:
<script>
function gtag_report_conversion(url) {
var callback = function () {
if (typeof(url) != 'undefined') {
window.location = url;
}
};
gtag('event', 'conversion', {
'send_to': 'AW-XXXXXXX/abcdefghijk',
'event_callback': callback
});
return false;
}
</script>