Från en Radio Times intervju med Steven Moffat, om hans sista episod som showrunner:
How far back did you plan to bring back the first Doctor? In the last series there seemed to be lots of references. For a start you named the companion Bill and her girlfriend was Heather like Bill Hartnell and his wife Heather.
Oh, that was a happy accident. Absolutely accidental.
So you didn’t name Bill after Hartnell – or even after your father, Bill?
No, if she’s named after anyone it’s that I liked the fact that, when we were doing The Day of the Doctor [2013], David Tennant called Billie [Piper] Bill. I thought that was a good name for somebody so I logged it then.