From the article:

"... an hour’s drive north from Shipton is a suburb of Rugby called Hillmorton, where at the time [the 1970s] the British government operated a very-low-frequency (VLF) radio station to send messages to submarines. It seems the powerful emanations from this nearby station, broadcast at a radio frequency of just 16 kilohertz (within the audio range), were picked up by the electronic equipment at [Richard] Branson’s studio [in Shipton] and recorded at a level too low for anyone to notice. After learning of this, I purchased an old CD of Tubular Bells, ripped a WAV file of one track, and piped it into a software-defined-radio package. Tuning to 16 kHz and setting the SDR software to demodulate continuous-wave signals immediately revealed Morse code. I couldn’t copy much of it, but I could make out many repetitions of VVV ('testing') and GBR (the station’s call sign)."