It's not any of the usual suspects, that's for certain. He'd have recognized Autons and there's not nearly enough yelling for it to be Daleks or Cybermen. To say nothing of the fact that the latter two would most likely have already made their presence known if they'd been around.

"I don't think so. Or if it is, they certainly aren't being very obvious about it. So either it's a very subtle invasion, or it's something like that time I ended up in the Land of Fiction."