The Doctor nods a bit. "Not that I've given up. But I've been here over two weeks now, and, well, progress is slow, and the TARDIS barely has power here. No time travel. Haven't really risked space travel yet either, because she gets power from the rift."

Should be safe telling the Master this, if only because he'll be in the same situation. He doesn't want the Master to try taking off and accidentally hurting his TARDIS.