I remember watching this episode live on first broadcast (1992) and feeling deeply short-changed by the plot. On re-watching it I was still disappointed by the outcome of the main crime investigation but found the episode surprisingly refreshing compared to some of the recent dire episodes in the series (not that that is saying much) and there was just enough mystery in the subsidiary plot to make it tolerable.
Away from Oxford and the familiar props (in all senses of the word) of the Jag, beer, dons, and record collection, Morse needs some imaginative zing to fill the gaps and the maintenance of his nauseating habit of panting over the most attractive female suspect does not count.
DoTS is just about saved purely by the visual appeal and suggested atmosphere of the Italian locations, the exotic intrigue of the Italian police (plenty of arm-waving and are they up to no good or just engaged on some great and secret line of justice ?), and a few hints at a car chase around tiny mediaeval streets.
So as a detective's foreign-excursion episode it scores highly over Promised Land (simply that DoTS makes one want to go to Verona whereas Promised Land merely confirms one's bafflement that anyone would want to go to Australia). BUT this is supposed to be a major murder mystery series, not "Wish You Were Here" (no doubt he would try to get off with Judith Chalmers if it were). If you have to rely on scenery to make a major who-dunnit watchable then things have come to a sorry state.
And surely the great John Thaw could have been told to sit through the final pan-out shot without staring at the camera !!!