Stacey's twin sister is named Angela and she works as a computer researcher at New Scotland Yard. Angela had a bit of an obvious crush on Detective Matt Devlin and was devastated by his death.
Jessica Gunning plays both roles - Stacey in the 'Doctor Who' episode "Partners In Crime" and Angela on a recurring basis in 'Law & Order: UK'. Based on admittedly scant research, Angela's last name has not been revealed on 'Law & Order: UK'. So for the time being I'm going to stand by this claim that her last name is Campbell until contradicted.