I had the relatively unusual experience of watching this dramatisation very shortly after listening to Ian Carmichael's reading of the audiobook of "Clouds of Witness", and was struck by the virtual absence of departures from the source material. As this is rare for TV/film adaptations, I do consider it worth mentioning. As with at least one other reviewer here, I found Lady Mary Wimsey's portrayal distracting at best, annoying at times, and unsure whether to blame actress or director, so will split the difference by mentally sharing responsibility between the two. Apart from the apparent age difference between Ian Carmichael and Lord Peter (which he himself notes on occasion), his performance of the role is extremely good, owing in large part, I suspect, to his familiarity with the source material and long championing of this series, of which I only recently learned. There is much to be said for labors of love, and clearly, this is one. The DVD set also includes "The Unpleasantness at the Bellona Club", plus an interview conducted in 2000 with Carmichael and additional information about his career and that of author Dorothy L. Sayers.
