Got it working by not using the_sub_field.. either way I would like to get to the bottom of this for future use. Many thanks.
Working script:
// Stage two
// For each field id store each fitters name in the posts array in a new array named $values.
foreach($fields_id as $fields){
$rows = get_field('fitters', $fields);
if($rows):
foreach($rows as $row) {
$values[] = $row['fittersname'];
}
endif;
}