Well that was a whopper of the first part of a Cyberman story. Like with World Enough And Time/The Doctor Falls, you really got the sense of the Cybermen as an unstoppable threat and no idea how Yaz and Graham are going to survive. Only criticism is that Jack really needed to be there, because his warning about the Lone Cyberman linked him too closely with the Cyberman story. He has to be in the eecond part, otherwise his warning makes no sense; you have to know about something in order to be able to warn about it in the first place! (NB: Jack did not return in The Timeless Children).