The Jensen Code

The Jensen Code was an adventure-drama aimed at the younger crowd. Found this info online:
Sixteen-year-old Terry Connor is sent, along with a few of his friends, to an Outward Bound centre. On his first day at the centre Terry is taken pot-holing by the senior instructor, Alex. All goes well until, at 100 feet underground, Alex goes to search for the torch that Terry has dropped. The Jensen Code was a highly unusual children’s thriller series, written by Carey Harrison (author son of actor Rex Harrison), and starring David (Dai) Bradley, the BAFTA-winning young star of Kes, as the teenager who unwittingly uncovers a terrifying space-age espionage project. This complete 13-part series, rarely seen since its original transmission in 1973.