If it's no bother. Try [a href="http://www.silverstripe.org/stable-download/"]downloading[/a] the latest SilverStripe version again. And see what happens :)

Thanks kiwidude95 but -- no change...

Cracked it! There mush have been a random space somewhere in page.php (dunno where but anyway)...

For the record correct page.php is:

<?php
class Page extends SiteTree {
	public static $db = array(
							  'SidebarHTML' => 'HTMLText'
							  );
	public static $has_one = array(
								   );
	function getCMSFields() {
		$fields = parent::getCMSFields();
		$fields -> addFieldToTab("Root.Sidebar",new HTMLEditorField("SidebarHTML"));
		return $fields;
	}
}
class Page_Controller extends ContentController {
	public static $allowed_actions = array(
	);

	public function init() {
		parent::init();
	}
}