Using silverstripe-linkfield with subsites

I have a silverstripe site with 6 subsites, and have used LinkField throughout for creating various links in banners etc, however the 'Page on this site' TreeDropdown shows all pages in the 6 sites, and I have tried various options but cannot get it to filter by subsite - any ideas?

Yes, that was me :) This was what I ended up with:

class SiteTreeLinkExtension extends Extension
{
    public function updateCMSFields(FieldList $fields)
    {
        $fields->removeByName('PageID');

        $currentSubsiteID = SubsiteState::singleton()->getSubsiteId();

        $fields->insertAfter(
            'LinkText',
            TreeDropdownField::create(
                'PageID',
                _t(__CLASS__ . '.PAGE_FIELD_TITLE', 'Page'),
                SiteTree::class,
                'ID',
                'TreeTitle'
            )->setSourceObject(SiteTree::class)
            ->setFilterFunction(function ($page) use ($currentSubsiteID) {
                return $page->SubsiteID == $currentSubsiteID;
            })
        );
    }
}