And Sandifer is right. This is a true gem to have uncovered. The two particular set-piece scenes that I had really hoped would work - Salamander entering his secret underground lair, and the confrontation between Salamander and the Doctor at the end of the story - more than lived up to my expectations. (Letts is particularly regretful about the latter in his memoirs, and there is perhaps an element of haste about it, but it is still pretty damn good.) More than that, this is a superb performance by Patrick Troughton, as two very different characters, each of which at various time pretends to be the other - I am a sucker for these blurred-identity yarns anyway, but Troughton takes this to a level that is not managed in any other Doctor-meets-his-double story.
Most of the rest is great too - Carmen Munro plays Fariah, possibly the most interesting non-white woman in the whole of Old Who (not a lot of competition - Ping-Cho and Shou Yuing are the only others who come to mind); the other cast are good too, both the above-grounders and the undergrounders; and even the third episode stands up way better in full context, with the comic Australian chef not quite so out of place when we know more about his environment. It's a story that has generally been a bit overlooked, as the only one of Season 5, the Monster Season, that lacked actual monsters; that will change now, as fannish wisdom adjusts to the newly revealed reality.