Problem when adding 'www'

Hello all,

I’ve just tried changing:
https://christmaspartymagician.co.uk
to
https://www.christmaspartymagician.co.uk
in General settings, but when I click save changes, I get this:

Warning: is_writable() [function.is-writable]: open_basedir restriction in effect. File(/) is not within the allowed path(s): (/home/[…]:/usr/lib/php:/usr/local/lib/php:/tmp) in /home/[…]/public_html/[…]/christmaspartymagician.co.uk/wp-admin/includes/misc.php on line 133

Warning: Cannot modify header information – headers already sent by (output started at /home/[…]/public_html/[…]/christmaspartymagician.co.uk/wp-admin/includes/misc.php:133) in /home/[…]/public_html/[…]/christmaspartymagician.co.uk/wp-includes/pluggable.php on line 890

Any ideas why?

Thanks