Jane Cain, the voice of the speaking clock, remembers making the first recordings:
The actual recording wasn't nearly as strenous as most people seem to think... The real work was done by the engineers of the Post Office, who had a wonderful place up at Dollis Hill, where they took the recordings and played about with film strip and the like.
The way I recorded it was in jerks as it were. I said: "At the Third Stroke" (that does for all the times), and then I counted from One, Two, Three, Four, for the hours, we even went as far as twenty-four, in case the twenty-four hour clock should need to be used, and then I said "...and ten seconds, and twenty seconds, and thirty, forty, fifty seconds", and "o'clock" and "precisely". The famous "precisely". So what you hear is "At the Third Stroke it will be one, twenty-one and forty seconds".
From an interview with Manchester Radio, 24 July 1957. Transcript held at BT Archives.

Picture location: London
Date: 1936
Sent by: Sian Wynn-Jones
Category: Communication instruments