I've managed to get the CHMOD working with the code posted below. This php web page incorporates an online wysisyg web page editor which allows the the user, through a password protected folder, to update an availability chart.
The form that displays the webpage to be edited has SAVE and BACKUP buttons which write the amended page to the live webpage which is displayed to any person accessing the availability chart from the website.
The following code sits at the top of the php file above the html code that displays the online editor.
One thing I got stuck on for a while was the different directory paths that are required for the ftp_site and the fopen. It may not be the best piece of coding but it does what I want in terms of protecting the files from unauthorised access.
If ($_POST['btn_name'] == "Backup")
$file = "/home/vps_username_name/public_html/availability_chart.bak";
$ftpstream = @ftp_connect('localhost');
//Login to the FTP server
$login = @ftp_login($ftpstream, 'ftp_username', 'ftp_password');
if($login) {
ftp_site($ftpstream, "CHMOD 0777 /public_html/availability_chart.bak");
$handle = fopen($file,'w');
fwrite($handle,$sContent);
ftp_site($ftpstream, "CHMOD 0644 /public_html/availability_chart.bak");
ftp_close($ftpstream);
}