At this stage TV ratings were published with only the Network Top 20, meaning programmes on the minority channel of BBC2 were extremely unlikely to feature. The release of Top 10s per channel began in 1981.
Des's interview with Ronnie Corbett is, like the majority of his interviews, particularly from this period, essentially an exercise in Des providing the cues for the guest to crack jokes. While such things may amuse, even if many sections are clearly pre-planned, it doesn't reveal a lot about the guest, but instead showcases their comedy.