The original sector clock for PFF HQ, which can be seen in this website's pictures of Bennett in his control room with his staff, is still treasured at RAF Wyton where it hangs on the wall in an entrance hallway full of historical photographs and pictures.
Sector clocks were invented prior to the start of the Second World War, and had several modifications as they adapted to war-time conditions. These highly accurate clocks were of particular use at the time of the Battle of Britain, when they were used to track enemy aircraft or intruders.
The following description of a sector clock is taken from the excellent display at Thorpe Camp museum. The museum is located near Woodhall Spa and Coningsby, home to 97 Squadron aircrew before and after the Bourn period, and contains much 97 Squadron material.
"Included on the clock in addition to the usual 12 hour dial was an outer ring of minutes numbered at five minute intervals, an inner ring extending the 12 hours to 24, and a repeated pattern of three colours. These colours were a stroke of genius. Initially set at 10 minutes per colour, a sighting placed on the display table at Sector HQ would be given a coloured disc appropriate to the time of the sighting. By the time the clock had moved on to the third colour the sighting marker with its old colour could then be removed. Bearing in mind intruders were only tracked by continuous fresh sightings this method kept the display table clear of redundant information and the Defence Control a good idea of 'time in the air' at a glance. "
Later the colours were reduced to five minutes, as can be seen in the PFF clock.
I am not entirely sure why this clock was in use at PFF HQ unless it was standard RAF issue for Group HQs and briefing rooms. Perhaps someone can enlighten me.