wp-admin gives 404 error

I can log into my site OK, but when I try to go to the dashboard, i.e.

http://www.extraconnections.co.uk/wp-admin/

I get a 404 error. Some dashboard pages, e.g.

http://www.extraconnections.co.uk/wp-admin/plugins.php

work OK, but others don’t. Pages that I’ve found searching on the web suggest that it could be to do with permissions – I have 644 for all files and 755 for all directories – or .htaccess, mine’s below:

redirect 301 /oracle/scripts.shtml http://www.extraconnections.co.uk/code/oracle-scripts
redirect 301 /oracle/array.shtml http://www.extraconnections.co.uk/code/array
redirect 301 /oracle/resources.shtml http://www.extraconnections.co.uk/articles/oracle-resources
redirect 301 /web/sites.shtml http://www.extraconnections.co.uk/portfolio
redirect 301 /web/password.shtml http://www.extraconnections.co.uk/code/password
redirect 301 /web/resources.shtml http://www.extraconnections.co.uk/articles/web-resources
redirect 301 /about.shtml http://www.extraconnections.co.uk/services
redirect 301 /web/sites.shtml http://www.extraconnections.co.uk/portfolio
redirect 301 /cgi-bin/contact.pl http://www.extraconnections.co.uk/contact
redirect 301 /cvw6.doc http://www.extraconnections.co.uk/cv.doc

redirect gone /oracle/
redirect gone /web/
redirect gone /cgi-bin/search.pl
redirectmatch gone /cgi-bin/yhaforum.pl.*

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_HOST} ^extraconnections.co.uk$ [NC]
RewriteRule ^(.*)$ http://www.extraconnections.co.uk/$1 [R=301,L]
</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

I’ve just upgraded to 2.8.5, but it was showing the same misbehaviour in the old version. I’ve tried (auto) reinstalling, but it doesn’t help.

Today’s the first time I’ve had this issue – it’s been working fine for ages (though I don’t log in much)