The holding has been secured through an equity and debt refinancing of the business led by its chief executive Simon Harbridge alongside the management team.
The deal sees private equity investor RJD Partners realise its investment which was acquired via a management buy-out (MBO) of Stone Group in 2008.
Some RJD investors will retain an interest in Staffordshire-headquartered Stone having invested alongside Souter Investments in the new ownership structure.
John Berthinussen, investment director at Souter Investments, said: “Stone Group presents an exciting opportunity for Souter Investments to back an experienced and successful management team, leading an established and significant technology business highly regarded in a fast-moving sector.”
Dickson Minto acted as legal adviser for Souter Investments.
Simon Harbridge, chief executive of Stone Group, said: “This transaction represents an important and significant milestone for Stone Group.
“Souter share our vision for the group and will be a strong and dynamic supporter for the next stage of our journey.”
Souter has key investments in some 25 businesses.