Import Plugin date format (Post Date postDate)
I haven't yet used the plugin, but I'd try to import the dates into another, custom datetime field (e.g. myImportDate) to see if it's a problem with postDate being a special field (also goes into other tables in the DB, as far as I know). This would be for testing purposes only...

Another hint that might help:
Craft usually saves a datetime in a format like so: "2014-07-31 12:00:00". I've no idea if this is relevant for your plugin though. Same applies for the timezone, as Craft saves a datetime in UTC. As the plugin uses the saveEntry API, you probably don't have to care about the timezone, but the string format could break it for you.

Edit:

OK,postDate should be possible and the timezone defined after Craft install should be used to save datetime values. I'd bet it's the unusual "31/07/2014 00:00" format, Simon!