Problem with ordering by a calculated field

I am trying to sort some query results using a distance field which is
calculated from longitude and latitude fields as follows:

$this->paginate = array(
'User' => array(
'recursive' => -1,
'limit' => 10,
'conditions' => $this->User->parseCriteria($this->passedArgs),
'contain' => array(
'Postcode' => array(
'fields' => array(
'latitude',
'longitude',
'(3963.0*ACOS(SIN('.$lat.'*PI()/180)*SIN(latitude*PI()/180)+COS('.
$lat.'*PI()/180)*
COS(latitude*PI()/180)*COS((longitude*PI()/180)-('.$lon.'*PI()/
180))
)) AS distance'
)
)
),
'order' => array('distance' => 'asc'),
)
);

"latitude" and "longitude" are fields on the Postcodes table. $lon
and $lat are variables. The problem is that the order by clause is
ignored, it does not appear on the SQL outputted in the debugger.
Also if I try and output the 'distance' field in the view using either
$user['User']['distance'] or $user['Postcode']['distance'] or
$user['distance'] I get an error:

Undefined index: distance [APP\View\Users\search.ctp, line 30]

Any ideas as how to solve this, a different approach perhaps?

Thanks.

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