In the BBC archives is a interesting series of documents and articles dealing with the The Genesis of Doctor Who | Creating a science fiction hero . I found it quite revealing on how some of the execs at the time viewed the good doctor and how the audience slowly grew over the years. It seems the series was not an overnight success . Matter of fact you will find that the series first 4 part mini series had a relatively small number but it was the second story with the Daleks that propelled the Dr into fame and fortune. That bit of trivia came form the Did you Know section of the Audience reaction report.
The first episode of ‘Doctor Who’ attracted a modest 4.4 million viewers, but the ratings did rise slowly as the four-part serial progressed. It was with the second story, however, that the series began to gain an ardent following. The introduction of those perennial villains the Daleks added over 6 million viewers to the ratings.