403 forbidden error can be caused by lack of index.php file on the root site directory. Check to make sure you have index.php. If not, make a new file with a file name Index.php and paste the code bellow:

<?php
$autoload_path = __DIR__ . '/vendor/autoload.php';
$autoload_available = include_once($autoload_path);
if (!$autoload_available) {
    die("Couldn't include '$autoload_path'. Did you run `composer install`?");
}

return \Elgg\Application::index();

Thank you for your reply.  I checked with my host provider and they didn't have any trouble viewing my sites - it was only me, looks like it was localised to my area (power outages during the night, disruptions to broadband).

Not sure why it ONLY effected my elgg sites, though - so weird.

I restarted my internet connection, as well as my devices, and after a while finally the sites are all accessible again.