Inside 55 Broadway...

Over the years the clock system has not been maintained as well as it might have been. Until the introduction of the Rugby clock receivers as standalone 'master clocks' all clock pulses were generated and derived at Leicester Square clock rack with subsidiary clock racks at Baker Street and IIRC also at St James's Park.

There were many traditional electromechanical master clocks on each line generating 'local' clock pulses but generally these were standby clock pulses as the main hourly sync and half minute clock pulses were repeated down the line from the main clock rack.
The hourly clock pulse for room 222 & BBMS came from a 60v style 'F' relay in the 'snakepit' beneath Leicester Square old exchange and unless someone has altered the arrangement the same relay still feeds the hourly sync pulse for BBMS from the NOC which I installed when it was opened. Interestingly I installed a Rugby clock receiver to supply half minute clock pulses to the BTP info room and NOC, these were just coming into vogue at the time but there is so much electrical interference within 55, Broadway that I recall having real difficulty finding a location in the BTP Comms equipment room that gave a reliable output.

When the control rooms were given centralised longline PA systems in the early 1980s the systems were also supposed to carry clock pulses to every station, this being achieved with 50 baud modems superimposing a signal over the PA control cable to drive the clock cards in each PA rack which in turn were to locally drive all the clocks on a site. In the event the 50 baud modems were found to be unreliable and were removed, the PA racks supplying local half minute clock pulse output from each rack's standby clock board as local standalone unsynchronised master clocks gradually replacing other clock systems.

With the introduction of the local Rugby radio clock units the 'Railway Time' black box time repeaters began to appear at many stations carrying the clock pulses from top station deep down to all platforms via a local distribution network. I assume that this remains the most up to date system though I don't know and would not be at all surprised to learn that timing is done in other ways such as synchronising to NTP time servers via the intranet.

As in all things LUL it takes a long time for some things to change and the clock system is one excellent example as many of the stations still display American self winders taking up to + or - 4 minute adjustments on the strike of the hourly pulse hammer to correct the time to the hour. AFAIK many old hourly sync, half minute, half minute bi-di and one minute clocks are still to be found all over the combine on series and parallel circuits alongside all the later systems but are probably being replaced and upgraded as they become faulty and beyond economical repair. The temptation of course being to take and old clock face and work it from modern technology, LUL has quite a bit of that around as well usually done in station modernisation works.