In case anyone else has found it hard to book guest flats, I recommend Easyhotel on Old Street as an affordable alternative. The rooms are as basic as the guest flats, but cleaner, newer and more comfy beds, so my parents tell me. And the rooms are cheaper than the guest flats.
Yes, Sam(uel) Michael Smith arrived safe and sound on Saturday 9th Feb. we're all doing well, though juggling 2 kids does seem to mostly be an exercise in minimising sleep deprivation!