Step 1:

Login to your billing account at http://order.cwcs.co.uk/billing/index.php

Click "Account" as highlighted in the image below and then click "Login"

Step 2:

Once you have logged in select "Domains" and then "My Domains" as highlighted in the image below:

Step 3:

You will now see all the domains associated with your account, click "Manage Domain" against the domain you want to edit the DNS for:

Step 4:

Click the "Nameservers" link

Step 5:

You will now see the screen to alter the nameservers for your domain name