ATT: CLYDE.

I think I've fixed it now.

In includes>>>extra_datafiles>>>testimonials_manager.php

the following appears at line 13 :

define('FILENAME_TESTIMONIALS_MANAGER', 'testimonials_manager.php');
?>

which I have edited to state:

define('FILENAME_TESTIMONIALS_MANAGER', 'testimonials_manager');
?>

and it seems to have done the trick.

I just hope it doesn't bugger up anything else! :)