Some strange sound mixing on last night's BBC4 episode as well, again something that presumably wouldn't have been noticed originally as it would have been in mono.
Though it's in stereo, it sounds to me like a mono soundtrack with one of the stereo channels inverted, a fake stereo effect basically. When then folded down to mono the sound gets quieter and tinny. This happened on another BBC Four showing of a TOTP episode a couple of years ago, which was posted about here - though that did fold down to mono without any problems.
Don't know whether they are stereo tests from the time or just a mistake on BBC Four's behalf when broadcasting the episode now - though we did get an episode in proper true stereo for the first time a few months back (even a few of the videos had stereo soundtracks) so it can't be long now until we start getting them permanently.