CREATE TABLE `postcodes` (
  `UID` int(11) NOT NULL auto_increment,
  `outward` char(4) NOT NULL,
  `inward` char(4) NOT NULL,
  `easting` mediumint(7) NOT NULL,
  `northing` mediumint(7) NOT NULL,
  `lat` decimal(11,6) NOT NULL,
  `long` decimal(11,6) NOT NULL,
  `NGR` char(9) NOT NULL,
  `grid` enum('osgb','osie') NOT NULL,
  PRIMARY KEY  (`UID`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=27602 ;

CREATE TABLE `outbound_stats` (
  `lat_avg` decimal(20,10) NOT NULL,
  `long_avg` decimal(20,10) NOT NULL,
  `lat_stddev` decimal(20,10) NOT NULL,
  `long_stddev` decimal(20,10) NOT NULL,
  `number` mediumint(8) unsigned NOT NULL,
  `outward` char(4) NOT NULL,
  PRIMARY KEY  (`outward`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

INSERT INTO `outbound_stats` SELECT AVG(`lat`) `AS lat_avg`, AVG(`long`) `AS long_avg`, STDDEV(`lat`) AS lat_stddev, STDDEV(`long`) AS long_stddev, COUNT(*) AS `Number`,`outward` FROM `postcodes` GROUP BY `outward`

SELECT `postcodes`.`outward`, `postcodes`.`inward`, `postcodes`.`lat`, 
`postcodes`.`long`,`outbound_stats`.*, 
abs(`postcodes`.`lat`-`outbound_stats`.`lat_avg`) AS `lat_offset`, 
abs(`postcodes`.`long`-`outbound_stats`.`long_avg`) AS `long_offset` 
FROM `postcodes` LEFT JOIN `outbound_stats` ON `postcodes`.`outward` = 
`outbound_stats`.`outward` WHERE `outbound_stats`.`number` >= 10 AND ( 
(abs(`postcodes`.`lat`-`outbound_stats`.`lat_avg`) + 
abs(`postcodes`.`long`-`outbound_stats`.`long_avg`)) > 
(`outbound_stats`.`lat_stddev`*3 + `outbound_stats`.`long_stddev`*3 )) 
ORDER BY (`lat_offset`+`long_offset`) DESC

Excel Magic:
="INSERT INTO `postcodes` VALUES (NULL,
"""&A6&""","""&B6&""","""&C6&""","""&D6&""","""&E6&""","""&F6&""","""&G6&""","""&H6&""");"