You don’t have permission to access options.php

Grasshopper, your search-fu is weak.

At first blush, one might theorize this is related to your host and mod_security.
Here’s one approach, courtesy of Whooami:

IF YOU DO NOT HAVE an .htaccess in your wp-admin/ directory:

create a text file on your desktop:

put the following inside it:

<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>

save the file.

Upload the file to your wp-admin directory.

Rename the uploaded file to .htaccess (with the .)

IF YOU DO HAVE an .htaccess in your wp-admin/ directory:

Edit it and add:

<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>

HandySolo, your .htaccess feng shui is good: it appears to have healed my woes.

the feng shui is mine.

So this directive stops the .htaccess directory inheriting the default mod_security settings.

Yes.

Surely this is always an issue for servers running mod_security?

Surely, it is not. I have 2 blogs on two diff. servers. Both have mod_security compiled into Apache, and neither has this problem. Hosts configure mod_security differently.

Is there a reason the WP installation doesn’t include a .htaccess file with that directive by default?

See above. Beyond that, its not WordPress’ job to override something like that. And frankly, I would pitch a Digg-sized fit if the developers ever thought to do such a thing.