Error setting up multisite

I have decided to turn an old blog into a multisite and when setting it up after install I receive the following error

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-login.php on line 362

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-login.php on line 374

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-includes/pluggable.php on line 682

Warning: Cannot modify header information - headers already sent by (output started at /home/dianec/public_html/wp-config.php:47) in /home/dianec/public_html/wp-includes/pluggable.php on line 881

What have I done wrong/what do I need to do to amend this error?

thanks