H.C. Clements was a company in the field of security systems. Their office was located in central London. H.C. Clements was secretly owned by the Torchwood Institute. Lance Bennett worked there as head of Human Resources and his fiancée, Donna Noble, was temporarily employed there as a secretary. (TV: The Runaway Bride)
History
In June 2007, Donna had the choice of working as a full-time secretary for Jival Chowdry or as a temporary secretary at H.C. Clements, a security firm. (TV: Turn Left) She chose the latter and on her first day there, Lance Bennett offered her a cup of coffee. They quickly developed a romantic relationship, at least for Donna, who pressured him into marriage.
Unbeknownst to Donna, Lance had been working for the Empress of the Racnoss for at least six months. At the Empress' behest, he dosed her with Huon particles in her coffee, intending for his Huon-saturated bride to provide the energy that the Empress needed to resurrect her race.
At Donna's wedding reception, when a Roboform Christmas tree began firing explosives at the wedding guests, the Tenth Doctor found that H.C. Clements was owned by the Torchwood Institute. The Doctor asked Lance to take him and Donna to H.C. Clements, wondering who controlled the firm since Torchwood One's dissolution in the Battle of Canary Wharf. The trail led them to a secret base under the Thames, where Lance and the Empress of the Racnoss revealed their plans. The Racnoss had fashioned a large web above the pit, where the body of H.C. Clements still hung. (TV: The Runaway Bride)
Technology and security
The Doctor, Donna Noble and Lance with the H.C Clements hi-tech transport. (TV: The Runaway Bride)
H.C. Clements, being a security firm, had its own range of security systems. The company was owned by the Torchwood Institute, which had bought it in 1984. The Doctor discovered a basement level not on the floor plans and the three travelled to it, finding themselves in a long tunnel that led to the Thames Barrier. There, the Doctor discovered a laboratory where Huon particles had been manufactured and stored in liquid form. It was linked by tunnel to the H.C. Clements building. (TV: The Runaway Bride)
Alternate timeline
Donna's World
If Donna Noble had worked for Jival Chowdry instead of H.C. Clements, she would have never met the Tenth Doctor. Lance and the Empress of the Racnoss would have found someone else to dose with Huon particles. When the Tenth Doctor flooded the Racnoss' base Donna wouldn't have been there to convince him to flee and he would have drowned too fast for him to regenerate. This would have resulted in devastating effects for Earth and the universe. (TV: Turn Left)