Bushstar

Actually, I have solved this but I am not sure it is best practice. In PluginB I have updated it to the following.

class PluginB {
    public function plugina_check() {
        // Check to see if plugin is active is true
        if (in_array('plugina/class-plugina.php', get_option('active_plugins')) {
            // Create mew object
            $new_example = new PluginA();

            // The next call throws Undefined variable $example
            $result = $new_example->check();
        }
    }
}

I assumed that the first objected created should be available to other plugins and that creating a second object would be unnecessary. I have now created two objects of the PluginA class.