Okay, I don't really understand this, I have now done another install and I get the same problem that you get again.
So, I once again manually edited .htaccess so that it looked like this:
### SILVERSTRIPE START ###
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !(\.gif)|(\.jpg)|(\.png)|(\.css)|(\.js)|(\.php)$
RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* sapphire/main.php?url=%1&%{QUERY_STRING} [L]
### SILVERSTRIPE END ###
Then I went to firefox and entered :
http://ssm.co.za/InstallerTest/testrewrite
And that worked fine.
So, I decided that the actual rewrite mod was working fine, something else is borked, so I just "force" continued.
BUT, then I came across the other problem, where, after the files are deleted, it keeps on trying to access check-php.php when accessing http://ssm.co.za.
I am using FireFox however, and after going and manually deleting all the cache'd files, accessing http://ssm.co.za worked properly.
Now I'm wondering if the other problem is not also firefox related.
i.e. If you're still doing this and using Firefox, try deleting your firefox cache files manually. (Deleting them from within firefox does not work properly). There are kept in F:\Documents and Settings\Stoke\Application Data\Mozilla\Firefox\Profiles\yx4szwtv.default on my computer.
Good luck!