Without looking at any sort of reference, I will say that two things could be wrong.
You are using the wrong absolute path to the .htpasswd file, or the passwords in the .htpasswd file are unreadable. :-) If you have direct access to a linux shell, use the htpasswd command to generate the proper user and password combination, if not use a free site on the web. I find that they sometimes don't work that way though.
Also change this:
/var/www/vhosts/numyspace.co.uk/web_users/./home/~unn_t005763S/public_html/assignment/.htpasswd
To this:
/var/www/vhosts/numyspace.co.uk/web_users/home/~unn_t005763S/public_html/assignment/.htpasswd
“The best thing about a boolean is even if you are wrong, you are only off by a bit.”