Hi again.

Things are slightly better after the re-install.
Pointing my browser at [website] shows "Index of /" and all the files and folders therein are shown below that title.

Initially, pointing my browser at [website]/admin showed a 404 message. I searched for the 'admin' folder and found it in the '/custom' folder.
In the 'environment.php' file, I set: $bigtree["config"]["admin_root"] = "http://[website]/custom/admin/";  but that also showed a 404 message.

I moved the 'admin' folder out of the '/custom' folder and placed it in the [website] root then amended the above line to  "http://[website]/admin/";
Pointing the browser at  [website]/admin now shows "Index of /admin" and all the files and folders therein are listed below that title.

I found that [website]/core/launch.php was not set to be executable. I set it to be executable but it made no difference.

If I point my browser at [website]/site I get the example site home page displayed but without any css etc., being picked up. Looking at the page source, this file expects to find the css at "http://[website]/site/css/site.css" but there is no such file at that location.

The possibility occurs to me that the installer for version 4.2.21 may be putting files and folders in the wrong places or has not correctly reflected the structure of the example site.

Any more ideas, please?

Thanks for your comments. I confirmed that mod_rewrite is ON and I have amended the virtualhost entries. The result is that I now get a listing of all the files at the webroot when I point my browser at the site. When I attempt to get the admin site I get a 404 error. Copies of all (I hope) the relevant bits are shown below.

ACCESS LOG ENTRIES
================
127.0.0.1 - - [05/Mar/2018:10:22:00 +0000] "GET / HTTP/1.1" 200 880 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.168 Safari/537.36 OPR/51.0.2830.40"
127.0.0.1 - - [05/Mar/2018:10:30:23 +0000] "GET /admin/ HTTP/1.1" 404 2178 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3282.168 Safari/537.36 OPR/51.0.2830.40"

VIRTUAL HOST ENTRY
===============
<VirtualHost *:80>
    ServerAdmin tony@atown.eu
    ServerName shrew3a.org
    ServerAlias nww.shrew3a.org
    DocumentRoot /var/www/shrew3a

    <Directory /var/www/shrew3a>
        Options -Indexes +FollowSymLinks +MultiViews
        AllowOverride All
        Require all granted

        SetEnv CI_ENV development
        RewriteEngine On
        RewriteBase /
        RewriteCond %{REQUEST_FILENAME} !-f
        RewriteCond %{REQUEST_FILENAME} !-d
        RewriteRule ^(.*)$ site/index.php/$1 [L]
    </Directory>

    ErrorLog /var/www/shrew3a/logs/error.log
    CustomLog /var/www/shrew3a/logs/access.log combined
</VirtualHost>

config.environment.php LISTING (MINUS THE START AND END PHP TAGS)
===================================================
    // Time Zone
    date_default_timezone_set("Europe/London");

    // Website Environment
    $bigtree["config"]["debug"] = full; // Set to false to stop all PHP errors/warnings from showing, or "full" to show all errors include notices and strict standards
    $bigtree["config"]["domain"] = "http://shrew3a.org";    // "domain" should be http://www.website.com
    $bigtree["config"]["www_root"] = "http://shrew3a.org/site/"; // "www_root" should be http://www.website.com/location/of/the/site/
    $bigtree["config"]["static_root"] = "http://shrew3a.org"/site/; // "static_root" can either be the same as "www_root" or another domain that points to the same place -i t is used to server static files to increase page load time due to max connections per domain in most browsers.
    $bigtree["config"]["admin_root"] = "http://shrew3a.org/site/admin/"; // "admin_root" should be the location you want to access BigTree's admin from, i.e. http://www.website.com/admin/
    $bigtree["config"]["force_secure_login"] = [force_secure_login]; // If you have HTTPS enabled, set to true to force admin logins through HTTPS
    $bigtree["config"]["environment"] = "dev"; // "dev" or "live"; empty to hide
    $bigtree["config"]["environment_live_url"] = "http://shrew3a.org/site/admin/"; // Live admin URL
    $bigtree["config"]["developer_mode"] = false; // Set to true to lock out all users except developers.
    $bigtree["config"]["maintenance_url"] = false; // Set to a URL to 307 redirect visitors to a maintenance page (driven by /templates/basic/_maintenance.php).
    $bigtree["config"]["routing"] = "[routing]";
    $bigtree["config"]["cache"] = false; // Enable Simple Caching
    $bigtree["config"]["cache_ttl"] = 300; // Number of seconds to keep a cached page
    $bigtree["config"]["sql_interface"] = "mysqli"; // Change to "mysql" to use legacy MySQL interface in PHP.

    // Database Environment
    $bigtree["config"]["db"]["host"] = "localhost";
    $bigtree["config"]["db"]["name"] = "[BLANKED]";
    $bigtree["config"]["db"]["user"] = "[BLANKED";
    $bigtree["config"]["db"]["password"] = "[BLANKED]";
    $bigtree["config"]["db"]["port"] = "";
    $bigtree["config"]["db"]["socket"] = "";
   
    // Separate write database info (for load balanced setups)
    $bigtree["config"]["db_write"]["host"] = "[write_host]";
    $bigtree["config"]["db_write"]["name"] = "[write_db]";
    $bigtree["config"]["db_write"]["user"] = "[write_user]";
    $bigtree["config"]["db_write"]["password"] = "[write_password]";
    $bigtree["config"]["db_write"]["port"] = "[write_port]";
    $bigtree["config"]["db_write"]["socket"] = "[write_socket]";
   
    /*
        Multi-Domain Support
        --------------------
        "trunk" should be set to the Page ID of the page that serves as the root-level page for the domain.
        The array key for each site will be defined as BIGTREE_SITE_KEY when you've been routed to that site.
        It should be alphanumeric as it is also used in tokenization of URLs.
   
        Example
        =======
   
        $bigtree["config"]["sites"] = array(
            "primary" => array(
                "trunk" => 0,
                "domain" => "http://primary.bigtree.dev",
                "www_root" => "http://primary.bigtree.dev/",
                "static_root" => "http://primary.bigtree.dev/"
            ),
            "alt" => array(
                "trunk" => 7,
                "domain" => "http://alt.bigtree.dev",
                "www_root" => "http://alt.bigtree.dev/",
                "static_root" => "http://alt.bigtree.dev/"
            )
        );
    */