Last column not imported

[mspacemedia]

I have been importing a dataset that looks like this:

Town	County	PostCode	Longitude	Latitude
Westdene	East Sussex	BN1 5	-0.16129	50.86153
Brighton	East Sussex	BN1 6	-0.13361	50.8431

However unless I add a further column to the right:

Town	County	PostCode	Longitude	Latitude	Extra
Westdene	East Sussex	BN1 5	-0.16129	50.86153	abc
Brighton	East Sussex	BN1 6	-0.13361	50.8431	def

The latitude column doesn't get imported into any records. Using the original ModelAdmin importer works fine with the same dataset...

The class looks like this

class Area extends DataObject {

    static $db = array(
        'Town' => 'Text',
        'County' => 'Text',
        'PostCode' => 'Text',
        'Longitude' => 'Text',
        'Latitude' => 'Text',
        'Sort' => 'Int'
    );

So nothing scary there and the import is added to a Page getCMSFields gridfield like this:

$fields->addFieldToTab('Root.Areas', GridField::create('Areas', 'Areas we cover', $this->Areas(), $gfc = GridFieldConfig_RelationEditor::create()));
        $gfc->addComponent(new GridFieldOrderableRows('Sort'));
        $importer = new GridFieldImporter('before');
        $gfc->addComponent($importer);

Any ideas?