If, in the early 60s, the BBC had made a John Cleese-style training video on how not to write, cast, direct, and act a piece of prime-time drama it might have looked very like this.
This episode's failings are nothing whatsoever to do with the age in which it was made, nor the technical limitations of TV at the time, nor the size of the budget. In any era, the rambling inarticularcy of the script, dearth of directorial momentum, and truly bizarre casting would mark it out as bad TV.
Anyone who has seen Renee Houston's film appearances knows she could not act and anyone who remembers her radio contributions to panel shows knows that she compounded her inability with an extremely ugly, uncouth arrogance and staggering lack of self awareness. Her casting here is beyond any rational explanation; it truly is that bad; but at least it is safely isolated in this breathtakingly duff episode and wasn't allowed to ruin one of the better ones.