He joined Luther as a consultant in 1998. Clients include the Adult Learning Inspectorate and the Better Regulation Taskforce. Before Luther he was twice elected president of the National Union of Students.
Trainer's exit comes as four other partners plan a management buyout of Luther from founders George Pitcher and Charles Stewart Smith (PRWeek, 9 September).
Ex-Britain in Europe campaign director Simon Buckby left Luther last month to set up Champollion.