When does Slow Horses return?

Slow Horses is a British spy thriller television series based on the Slough House series of novels by Mick Herron. It follows an MI5 unit where disgraced or failed agents are consigned, under the supervision of Jackson Lamb (Gary Oldman) It also stars Jack Lowden, Kristin Scott Thomas, Jonathan Pryce, Saskia Reeves, Rosalind Eleazar, Christopher Chung and Aimee-Ffion Edwards.
Slough House is an administrative purgatory for MI5 service rejects who have bungled their job but have not been sacked. Those consigned there are known as "slow horses" - a pun on "Slough House" They are expected to endure dull, paper-pushing tasks, along with occasional mental abuse from their miserable boss, Jackson Lamb.