[eluser]diywm[/eluser]
I had not realised the key name to the array would be something like 'Tables_in_blogdb' - thanks for pointing this out. It turns out that because I am doing a like, I have to build the key to be like this, where the like parameter forms part of the key : Tables_in_blogdb (b00043%)
$sql = "SHOW TABLES LIKE '".$tableprefix."%'";
$query = $this->query($sql);
foreach ($query->result_array() as $row){
$tablename = $row['Tables_in_blogdb ('.$tableprefix.'%)'];
echo tablename;
}