Thanks Slowhatch!
The course for The Big Cheese was 1.7km/1.1mi. Derek/Claire ended up on the next street while looking for the Osteria del Cappello, but this wouldn't have cost them more than a minute.
Glenda/Lumumba, however, went 0.75km/0.45mi out of the way from the Arco del Meloncello, to the junction of Via XXI Aprile 1945 and Via Antonio Zannoni. It's such a distance to the centre of town, the lady they were asking told them not to walk, but to take the bus instead!