As soon as UNIT comes into the equation, "continuity" loses any vestigial semblance of meaning anyway. I'd rather like to see some stories which really stretch UNIT continuity. Having the usual UNIT crew going about their usual business in an 18th century setting without a word of explanation, for example.
— Daniel Gooley, on rec.arts.drwho, 11 January 1999.
So, by the magic of UNIT dating, this story is set in 1793.