I wouldn't say BtE is Series IX. It is the ninth series though. "Isn't that the same thing?" you may ask. No doubt I'm splitting hairs, but what I'm referring to here is the title of the series which is, Back To Earth. Either way, being the ninth series (and three episodes, while really a mini-series as we count this, is still a series in the literal meaning of the word, more than 2), it follows that the tenth series is Series X. Of course I'd consider the Director's Cut of BtE as the true version... and that's one long episode... but it's still in ninth place. On the other hand, it's just titles, and one is good as another depending on how you'd like to interpret things. What does it matter?