Paul Darrow also said this in an interview once:
PAUL: At the end of the series Avon was left standing, and Terry had this idea, that he'd come back like Napoleon after Elba, but he's going to loose. The last 100 days, but he puts up a good fight. And he's provoked into coming back. He's quite happy where he is, he's been there for twenty years, but suddenly a new government has come in, a new Federation and they are going through the computers and someone says, "What happened to him?" and the other guy says, "Oh we killed Blake." So they go on a bit further and then he says, "And what happened to this one?" "Oh dear! We forget about him," and so they go and get him and of course they can't get him, and that kicks it off. That was the idea, basically. And we are taking it from there. We've developed it in a slightly different direction.