The Reference Desk/Terror of the Zygons

From Tardis Wiki, the free Doctor Who reference

Just finished watching this great story which ends with the Doctor, Sarah Jane, Harry, the Brig and the (real) Duke walking to the parked TARDIS in the forest. Now, if anyone could enlighten me, how is it that the Duke knew the TARDIS' name, even more so what it was? As far as I know, he'd just come out of the Zygon ship earlier that day!