Horrocks, chief investment officer and managing partner at Blue 449, has been named managing director, trading of Publicis Media Exchange (PMX), "with responsibility for the UK’s trading division across the group".
John Heather, head of AV trading at sister agency Zenith, has been promoted to AV, trading director at PMX.
He will be responsible for all buying of audio-visual media including TV and video-on-demand. Heather will report to Horrocks.
The duo will effectively share much of the responsibilities that Locke held as managing director of PMX.
Publicis Media has not appointed a direct successor to Locke since his exit after 27 years earlier this year.
However, Simon Pardon, the global chief executive of PMX, has taken on an additional role as interim UK lead of PMX. Horrocks reports to Pardon. A permanent UK lead of PMX will be appointed later.
Insiders say Sue Frogley, president of global solutions at Publicis Groupe, has also been playing a growing role in the London media operation in a bid to drive the direction of Publicis Media.
Parent company Publicis Groupe set up Publicis Media at the start of 2016 to bring together the group’s four global media agencies, Zenith, Starcom, Mediavest and Blue 449, after a string of major US account losses.
Publicis Media, the world’s second biggest media buyer behind WPP’s Group M, manages more than £1bn of billings a year in the UK for clients such as Asda and Mars and uses PMX to leverage its buying power on behalf of its agencies.
Publicis Media has made a string of other appointments as part of the trading reorganisation.
Chris Hayward, previously AV negotiation director, has been made strategic adviser to PMX.
That follows the appointment last month of Ben Chesters, who was promoted from managing partner, precision marketing at Starcom to managing director, investment operations at PMX, with responsibility for all agency relationships.
Pardon said "the centralisation of key resource" in PMX "is part of our on-going transformation to deliver even stronger investment options for our clients". Each of the four agencies will also have its own head of investment as part of the reorganisation, Pardon added.
David Mulrenan will take the role at Zenith and Oliver Ruff will do the job at Mediavest. Malcolm Boxall already holds the title at Blue 449 and Starcom will make an appointment in due course.
Publicis Media has insisted it does not want to dilute the strength of its agency brands, despite centralising buying, analytics and other areas in a "matrix" structure within the group.
Pardon said the appointments across PMX and in the agencies will "leverage the incredibly strong and experienced team, bringing real depth and expertise across all areas".
Amanda Morrissey, who was previously general manager of AKQA London, was named the first UK chief executive of Publicis Media a year ago.
Publicis Media said Frogley "is working with Amanda on a six-month project basis to help streamline the UK practices".