I've had the exact same problem, which after 4 days plusnet were not able to resove or suggest and answer.
I have solved this myself!
First create a index.html home page for your domain that you wish to redirect (domainb to be redirected to domaina), so create this page for domainb.
In this html page, include the following:
<body>Please follow <a href="http://www.domaina.com/">link</a>!</body>
This will have the result of "hiding" domaina from the user, as the browser will show www.domainb.com
in the address bar.
Hope this helps!!