I enjoyed this tremendously. The Doctor, Ace, and Bernice Summerfield, in nineteenth-century London, get mixed up with Sherlock Holmes and Dr Watson; and all five of them are then confronted with an invasion of Earth by the forces of Azathoth from the planet Ry'leh (sic). Mixing the mythoses (mythoi?) of Arthur Conan Doyle and H.P. Lovecraft is risky, but Lane has done it very well - lots of borderline steampunk in his Victorian settings, most of the narrative told in the first person by Watson (who inevitably develops a liking for Benny), cameo appearances from Pope Leo XIII, the San Francisco fire of 1906, and the smart missiles from Iain M. Banks' Culture novels.
Apart from the wonderful romp of the setting, Lane is also pretty smart about reinforcing our willing suspension of disbelief. Is Sherlock Holmes real or fictional in the Whoniverse? We get a rather neat answer here. On top of that, the entire narrative is nicely presented as a flashback, Benny and Ace perusing Watson's account, and then critiquing him as an unreliable narrator.
Strongly recommended, especially for fans of Holmes or Cthulhu who may for some reason not have encountered Doctor Who.