WLW recognises my Drupal 6 site, can download the theme, access old posts etc.

When I try to post (new or an edited old one), I receive the following error in WLW:

Network Connection Error
Error attempting to connect to blog at:
http://site/xmlrpc.php
The operation has times out

If I check the site Recent Hits, I receive:

12/03/2009 - 11:30 WLW Manifest
blogapi/wlwmanifest.xml Anonymous (not verified) details

The Recent Log Entries displays a matching 403.html record.

There appears to be some sort of authentication issue which is stopping me from posting. This is strange as the test post and reading of old posts seems to work fine.