What is the accuracy of the 'speaking clock'? (FAQ - Time)
The telephone 'speaking clock', the 'Timeline' service provided by BT with sponsorship from Accurist, gets its seconds markers from an accurate source of time. Even allowing for the delay in the telephone network, you can probably expect the starts of the seconds pips to be accurate seconds markers within about one-tenth of a second.
However, the labelling of these seconds is not controlled by, neither is it monitored by, NPL. If the 'speaking clock' appears to be out-of-step with other time signals it would be appropriate to take the matter up with BT.