Hi Klaus,
Thanks for the help and suggestion. I couldn't see anything in the error log file, but then inspiration struck. I have a .htaccess file set up to deal with some general issues and SEO things (canonicalization, url rewriting etc.). Here is an excerpt:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^websanity\.co\.uk [NC]
RewriteRule ^(.*)$
http://www.websanity.co.uk/$1 [L,R=301]
RewriteCond %{THE_REQUEST} (index\.php) [NC]
RewriteRule ^(.*)index\.php$
http://www.websanity.co.uk/$1 [L,R=301]
RewriteRule ^index\.htm$ /index.php [R=301,L]
RewriteRule ^index\.html$ /index.php [R=301,L]
I temporarily killed the .htaccess file and then suddenly the system sprung into life again. So, I just put overriding .htaccess file in the WB directory turning off the rewrite engine and that sorted it out on a permanent basis. Yipeee!!

Now, I'm no expert in .htaccess, and to some of you there might seem to be something stark staringly stupid about the above snippet, but it's something I've grabbed out of an SEO book at some point in the past, and this is the killer line that has never before caught me out:
RewriteRule ^(.*)index\.php$
http://www.websanity.co.uk/$1 [L,R=301]
Seems to coming into effect a bit too aggresively, so
RewriteRule ^index\.php$
http://www.websanity.co.uk/$1 [L,R=301]
sorts it out.
fairly obscure, but one to look out for.