There are several issues with the code you provided.
First, your json text is stored in $json, but you try to decode $xmlresponse. I guess that's just a copy/paste error, though.
Second, you try to access the surname using the object syntax although you explicitly force json_decode to decode objects as associative arrays.
Third, the json provided encodes an object in an array in an array. You ignore the nested structure of the response.
Try this:
$json='[[{"transTime":"2013-10-23 17:30:42","Forename":"Ian","Surname":"Graham","Address Line 1":"RG412GX"}]]';
$response = json_decode($json);
$displayName = $response[0][0]->Surname;
echo $displayName;
$xmlresponse?print_r($obj2);?