I never saw this particular television version, but I can tell you a little about it.
E. Philips Oppenheim was one of the popular spy novelists of the teens of the 20th Century. He seems to have had a reasonably good writing style, and he told a good tale of intrigue. THE GREAT IMPERSONATION was written in 1913, and was part of a series of novels that were about plots by a foreign power (actually Wilhelmine Germany) against Great Britain. The most notable one (besides this one) was Erskine Childers' THE RIDDLE OF THE SANDS, one of the best novels about sea espionage ever written. However, the genre was also spoofed. P. G. Wodehouse wrote a spoof about how seven countries try to invade Britain, and are defeated by the boy scouts. H. H. Munro ("Saki") told a typically cynical tale, WHEN WILLIAM CAME, about a successful German invasion - and showed how the upper crust in Britain would quickly become quislings! Oppenheim's tale has been filmed before - there is a film version starring Ralph Bellamy. The story tells how Baron Leopold Von Ragastein, a big game hunter and German spy, is on safari in Africa, and runs across a derelict: Sir Everard Dominey. Dominey is a drunken wreck who has fled his wife and family and position - and Von Ragastein is aware of two points about him. First, Dominey's position is a high one, with very close ties to the top echelons of Britain's political, diplomatic, and military worlds. Secondly, Dominey (when cleaned up) looks almost totally like Von Ragastein. He talks it over with his assistants, and they agree that it would be better to let the German continue alone with the drunken Englishman. Who knows what might happen afterward? So we see them going off by themselves alone on the safari. About half a year later, London society is amazed to find Sir Everard has reappeared. He is back to normal - no trace of his alcoholic haze. He is soon resuming his high level work in the government. But he remains estranged from his wife, who can't understand why. He is also haunted by a rumor that he may have murdered a man who fell in love with Lady Dominey. The man's mother is constantly confronting Sir Everard about this, and he is distant with her.
In the meantime, Von Ragastein's assistant has gotten placed into the German Embassy in London. He constantly approaches Sir Everard at diplomatic dinners and the like, and is received courteously, but he too finds Sir Everard somewhat distant. There are moments when he appears to be close to passing information, but he backs off (or the authorities seem to get wind of what might be going on).
So the reader is kept guessing until the end of the story about the secrets of the novel: Did Sir Everard commit a murder? Is it Sir Everard or Baron Leopold who has everyone so confused? And I will leave it at that - you may find the book and read it: An edition was published twenty years ago by Dover Books.