Use a second ORDER by in MySQL query
ORDER BY
can be a comma separated list. Just list them by order of presdence:
SELECT * , ( 6371 * ACOS( COS( RADIANS( latitude ) ) * COS( RADIANS( 51.61062 ) ) * COS( RADIANS( - 0.236952 ) - RADIANS( longitude ) ) + SIN( RADIANS( latitude ) ) * SIN( RADIANS( 51.61062 ) ) ) ) AS distance
FROM `questions`
WHERE `expiry_date` >
CURRENT_TIMESTAMP HAVING distance <=50000
ORDER BY distance
, expiry_date DESC
LIMIT 0 , 15