The show is particularly poorly-preserved in the archives: although the first season survives intact, there are very large gaps in the known holdings. Of the 191 editions, it is thought that only 56 are still known to exist; of these some installments no longer survive in their original format (e.g. there are only monochrome film prints of two color videotaped stories). Only seven of the later episodes exist in color.
Between March 1970 and December 1978, the three main actors (plus various supporting cast members) recreated their roles in a BBC Radio 4 adaptation of the series. For the first six series, TV scripts were cut down and adapted for the half-hour running time; from 1976 a further forty were specially written for the final three runs. As with the parent television series, the survival rate is very hit-and-miss.