Custom post type archive page template

I have managed to resolve this by adding in a custom rewrite into the functions file:

// Add custom rewrite rules to handle things like years in custom post archives
function add_rewrite_rules($aRules) {
    $aNewRules = array(
        'news/([0-9]{4})/([0-9]{2})/page/?([0-9]{1,})/?$' => 'index.php?post_type=clientnews&year=$matches[1]&monthnum=$matches[2]&paged=$matches[3]',
        'news/([0-9]{4})/([0-9]{2})/?$' => 'index.php?post_type=clientnews&year=$matches[1]&monthnum=$matches[2]',
        'about-us/other-news/([0-9]{4})/([0-9]{2})/page/?([0-9]{1,})/?$' => 'index.php?post_type=othernews&year=$matches[1]&monthnum=$matches[2]&paged=$matches[3]',
        'about-us/other-news/([0-9]{4})/([0-9]{2})/?$' => 'index.php?post_type=othernews&year=$matches[1]&monthnum=$matches[2]'
    );
    $aRules = $aNewRules + $aRules;
    return $aRules;
}

// hook add_rewrite_rules function into rewrite_rules_array
add_filter('rewrite_rules_array', 'add_rewrite_rules');

Then in the archive page I changed the query to:

$aParts = explode( "https://wordpress.stackexchange.com/", $_SERVER['REQUEST_URI'] );

$iYear  = get_query_var('year');
$iMonth = get_query_var('monthnum');

if( $iMonth <= 0 && $iYear > 0 )
{
    $iMonth = $aParts[ 4 ];
}

$news = new WP_Query('showposts=6&post_type=othernews&paged='. $paged . '&year=" . $iYear . "&monthnum=' . $iMonth );