The cache can be visited by parking in the Paulsgrove estate below the quarry, but a much better approach is to park on top of Portsdown hill at N50 51.487 W001 06.769 (room for 3 or 4 cars) where there are magnificent views to the south. From here you can visit '150,000 tons of fuel oil' before walking east along the road, over the tunnels of COHQ under Fort Southwick, as far as the Reservoir building. Following the footpath path downhill from here brings you to the quarry with more impressive views. En-route to the cache you will also pass the Cooper Shelter tunnels, very obvious in the quarry face. (Full details of the walk, with maps, can be found on the Portsdown Tunnels Web site. as Walk number 2).
The underground radio station is part of the COHQ complex beneath Fort Southwick. The invasion of Europe (D-Day landings) was controlled from here. The radio station was placed half a mile from COHQ so that the aerial array did not give its position away.
A full description of this, and Coopers Air Raid Shelter, the other underground feature of the quarry can be found at the Portsdown Tunnels Web site.
When the M275 was constructed the floor of the quarry was excavated for construction material leaving the entrances 40ft up the cliff face. If you are sure footed and not afraid of heights it is possible to explore the radio station tunnels by scrambling up the slope above the cache. A torch, boots and old cloths would be needed for this.