Thanks Jason, - i couldn't get the demo to work, but I got the idea.
I used the url you provided - so I can retrieve a json file using co-ordinates, and from the array, i can recover a postcode.
One issue i came across is that the array generated from the json does not always seem to have the same number of elements,, is there a way to always be able to select a postcode ?
eg, in this code snippet
$geoCodeURL = "http://maps.googleapis.com/maps/api/geocode/json?latlng=".$lat.",%20".$long."&sensor=true";
$result = json_decode(file_get_contents($geoCodeURL), true);
$_SESSION['retrievedpostcode'] = ($result['results']['address_components']['long_name']);
gives me the postcode, for a particular UK address, but this array value (4) is not a postcode on the url you provided
Other than this, i have a working example, just needs a bit of tidying