The trailer I've embedded above is 100% fake, but it's one of the best fakes I've ever seen. It appears to mix body doubles and CGI, so isn't solely a matter of re-editing existing footage together. I only hope the actual 50th anniversary special of Doctor Who is as epic and exciting as this fan-made teaser. It's even in stereoscopic 3D, if you have a pair of those retro glasses to hand. Well worth a look!