He watches Jack loading his gun and briefly wonders if he can ask to borrow one, but he's not sure that will go over with Rose, or really, with his own conscience. However, that doesn't mean he won't hurt the Master. Not by any stretch.
"Right. I was just getting a position, and then suddenly the phone was moving, awfully fast, so I'm pretty sure he chucked it at a passing lorry or something," he takes a breath, "but I've got it narrowed down to just two blocks by two blocks..."