Can't delete annotations via Web interface?

I am having some difficulty in deleting annotations through the Web 
interface at http://annotest.w3.org/annotations?explain=false .

I scroll down to 'Query Annotations', enter a URL next to "uris to check 
for annotations:" and click "query RDF DB". Amongst the output, I get an 
annotation starting

<a:Annotation r:about="http://annotest.w3.org/annotations/annotation/...">

I copy and paste this URL into the "Annotation ID" box under "Delete an 
Annotation". I leave the URI for the RDF namespace alone (it doesn't seem 
the kind of thing I want to customise!) and select "Delete annotation".

After a bit I am asked for a username and password for annotest.w3.org, 
realm "su". Entering my usual username and password combination doesn't 
work. Eventually I get an "Auth Required" error.

Is this intentional? Or am I doing something wrong?

Matthew Wilson