Symbiosis has restrictions in place to prevent people relaying on "routable" IP addresses, i.e. everything other than 127.0.0.1, and the other addresses in RFC1918.
If you're running this PHP on the same machine as the mailserver, you should connect to "localhost" rather than the name of the machine (mail.shed5.co.uk in this case) and that will allow you send mail.
If not, you can add the IP address of the server where the PHP is running to the
/etc/exim4/relay_from_hosts file, to allow unauthenticated relaying.
Otherwise, you'll need to configure a specific mailbox account for this purpose, and log in with credentials. Additionally, Symbiosis encourages encryption so the PHP script will have to use an encrypted connection to log in.