<script type="text/javascript">
(function() {

window.onload = function() {
 var mc;
// Creating an object literal containing the properties we want to pass to the map
var options = {
zoom: 10,
center: new google.maps.LatLng(52.40, -3.61),
mapTypeId: google.maps.MapTypeId.ROADMAP
};

// Creating the map
var map = new google.maps.Map(document.getElementById('map'), options);

// Creating a LatLngBounds object
var bounds = new google.maps.LatLngBounds();

// Creating an array that will contain the addresses
var places = [];

// Creating a variable that will hold the InfoWindow object
var infowindow;
mc = new MarkerClusterer(map);
<?php
$pages = get_pages(array('child_of' => $post->ID, 'sort_column' => 'menu_order'));
$popup_content = array();
foreach($pages as $post)
    {
    setup_postdata($post);
    $fields = get_fields();
    $popup_content[] = '<p>'.$fields->company_name.'</p><img src="'.$fields->company_logo.'" /><br /><br /><a href="'.get_page_link($post->ID).'">View profile</a>';
    $comma = ", ";
    $full_address = "{$fields->address_line_1}{$comma}{$fields->address_line_2}{$comma}{$fields->address_line_3}{$comma}{$fields->post_code}";
    $address[] = $full_address;
    }
wp_reset_query();
echo 'var popup_content = ' . json_encode($popup_content) . ';';
echo 'var address = ' . json_encode($address) . ';';
?>

var geocoder = new google.maps.Geocoder();

var markers = [];

// Adding a LatLng object for each city
for (var i = 0; i < address.length; i++) {
    (function(i) {
        geocoder.geocode( {'address': address[i]}, function(results, status) {
            if (status == google.maps.GeocoderStatus.OK) {
                places[i] = results[0].geometry.location;

                // Adding the markers
                var marker = new google.maps.Marker({position: places[i], map: map});
                markers.push(marker);
                mc.addMarker(marker);

                // Creating the event listener. It now has access to the values of i and marker as they were during its creation
                google.maps.event.addListener(marker, 'click', function() {
                    // Check to see if we already have an InfoWindow
                    if (!infowindow) {
                        infowindow = new google.maps.InfoWindow();
                    }

                    // Setting the content of the InfoWindow
                    infowindow.setContent(popup_content[i]);

                    // Tying the InfoWindow to the marker
                    infowindow.open(map, marker);
                });

                // Extending the bounds object with each LatLng
                bounds.extend(places[i]);

                // Adjusting the map to new bounding box
                map.fitBounds(bounds)
            } else {
            alert("Geocode was not successful for the following reason: " + status);
            }

        });

    })(i);

}
var markerCluster = new MarkerClusterer(map, markers);
}
})
();
</script>