Anneke Wills, who can't sing, ad libbed "But I can't sing", and shouted the words to "Auld Lang Syne" instead.
Everyone is dressed in fancy costumes to celebrate New Year's Eve, and Honor Blackman as Cathy addresses Jane wearing a pussy cat outfit as "Pussy"; a year later James Bond was calling Honor "Pussy" in her classic role as Pussy Galore in Goldfinger (1964).
An extra telecine insert in the script at 17:28 was dropped, probably because it was getting repetitive and making the episode run over.