<?php
$request = array( 'op' => 'edit'
                ,'contact_label' => 1
                ,80 => 'on'
                ,79 => 'on'
                ,76 => 'on'
                ,74 => 'on'
                ,73 => 'on'
);



$enquiries = array();
foreach($request as $key => $value) {
    if(is_numeric($key)) {
        $enquiries[] = $key;
        unset($request[$key]);
    }
}

$request['enquiries'] = $enquiries;

echo "<pre>";
print_r($request);
?>

<pre>Array
(
    [op] => edit
    [contact_label] => 1
    [enquiries] => Array
        (
            [0] => 80
            [1] => 79
            [2] => 76
            [3] => 74
            [4] => 73
        )

)