htaccess does not seem to be working

I have a site that I am trying to divert non-www to www. I can get this code to work on another domain I have, but it simply doesn't work on this one.

Here is the contents of the entire htaccess file.

Options +FollowSymlinks
RewriteEngine on
rewritecond %{HTTP_HOST} ^domain.co.uk [NC]
rewriterule ^(.*)$ http://www.domain.co.uk/$1 [L,R=301,NC]

As I say it works elsewhere but not on this domain.

I have tried placing the htaccess file in several different places but to no avail.

The server is a standard LAMP configuration - or so I am told.

Any ideas?

Thanks

Mike