OK, here's what was causing the problem:
the .htaccess file at the root level of the site was interfering with the way that WebDAV worked. Here's the contents of the .htaccess (and no, I didn't program this):
Options +FollowSymLinks
<IfModule mod_rewrite.c>
RewriteEngine on
# fix missing trailing slash
# -> the file has no extension
RewriteRule !..{2,6}$ - [C]
# -> and does not end with a slash
RewriteCond %{REQUEST_URI} !^.*/$
# => redirect
RewriteRule ^(.+)$ /$1/ [R=301,L]
RewriteRule ^(EN|FR)/([^>.)/([^>.*)/index.html(.)$ /index.php?s=$1&l=$2&p=$3 [NC,L,QSA]
RewriteRule ^(EN|FR)/([^>.)/([^>.)/$ /index.php?s=$1&l=$2&p=$3 [NC,L,QSA]
RewriteRule ^(EN|FR)/([^>.)/index.html(.)$ /index.php?l=$1&p=$2 [NC,L,QSA]
RewriteRule ^(EN|FR)/([^>.*)/$ /index.php?l=$1&p=$2 [NC,L,QSA]
RewriteRule ^(EN|FR)/index.html(.*)$ /index.php?p=$1 [NC,L,QSA]
RewriteRule ^(EN|FR)/$ /index.php?p=$1 [NC,L,QSA]
</IfModule>
php_value postmaxsize 55M
php_value uploadmaxfilesize 10M
ErrorDocument 404 /erreur404.html
I would have liked to fix the problem by overriding the parent .htaccess, but I couldn't quite get that working so I ended up moving to a different subdomain.
If anyone has a quick fix for overriding the parent .htaccess for WebDAV use, I would be interested.
Miles