This structure was completely below ground with raised entrances that would have been level with the surface at the time of construction. Subsequent development raised ground-level, totally obscuring any trace of the structure. As a result of this, shelter F3 was damaged by contractors during their sub-surface excavations and it was not possible to obtain complete measurements for the two entrances. Once shelter F3 was located and fully exposed, it was observed that the shelter was roughly aligned north to south. The northern entrance was 55cm above the roof of the chamber and 1m wide, with a surviving length of 5.2m descending at least seven steps into a corridor. The southern entrance was 45cm above the roof of the chamber and 1m wide, with a surviving length of 6.02m. Here the damage was only minor and the entrance descended eleven steps into a corridor. These had both been capped by concrete-covered corrugated iron that sat on a lip in the entrance walls that was 5cm deep at both the sides and chamber end. Both corridors were 70cm wide and terminated in recesses that were 87cm deep and contained a centrally-located exhaust vent (10cm in diameter). The floors in both corridors were of poured concrete with five 60cm by 46cm slabs set into them. A white painted number (50) was on a facing panel of each entrance.
The central chamber was identical to shelters F1-F2 in terms of measurements and fittings. <1>