Apache2 throwing 500 error for php files. log shows Premature end of script headers

Apache2 throwing 500 error for php files. log shows Premature end of
script headers

I'm on a relatively fresh install of Ubuntu and this is the first time
I've used the local web server on it.
The problem I have is that when I try and access a php file I get a 500
error.
To try and solve it I've: - Renamed index.html to index.php in /var/www to
make sure it's not a virtualhost error (standard "it works" file) - given
0777 permissions to both /var/www and the index.php file - made www-data
the owner of the directory and the file - made www-data the group for the
directory and the file - checked the error log - trawled Google
The error log shows: [Sat Sep 07 13:18:56 2013] [notice] Apache/2.2.22
(Ubuntu) PHP/5.4.9-4ubuntu2.3 configured -- resuming normal operations
which I understand means the PHP is loading correctly
The only entry I do not understand is [Sat Sep 07 13:19:00 2013] [error]
[client 127.0.0.1] Premature end of script headers: index.php I've done
searches but most of the articles I've found are no help.
Is there anything else I can do to try to debug the error or does anybody
have any ideas as to why this error is happening?
Thanks in advance, John