Given the error message:
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘tl_spam_free_wordpress_comments_form’ was given in /home/euanmitc/public_html/wp-includes/plugin.php on line 403
The error is being generated by whatever Plugin defines this function:
tl_spam_free_wordpress_comments_form()
…which appears to be Spam Free WordPress Comments.
This bit:
First argument is expected to be a valid callback
…generally refers to an add_action() or add_filter() call. For some reason, WordPress doesn’t recognize tl_spam_free_wordpress_comments_form() as a valid callback – which means that WordPress isn’t finding it as a declared (defined) function.
Do you have the Spam Free Plugin activated?