This was SOE Mission in Algiers responsible for organising the operations in the First Army area in Tunisia. It was set up under the command of Lieutenant Colonel Young in November 1942.
There was a plan to incorporate a part of the Small Scale Raiding Force (SSRF) into the Brandon Mission, which may be why Major Gwynne joined February 1943. However, due to conflict between various Special Forces commanders, this never took place and the SSRF men variously joined the SAS or SBS as these units were reorganised around this time.
Operations included seaborne missions from both boats and submarines as well as parachute drops. One of the naval insertions, Operation Felicity, resulted in the capture of former Auxiliary Units, Officer Captain Crosthwaite-Eyre.
The Mission was dissolved in May 1943 on the conclusion of the Tunisian campaign.
It should be noticed that the Massingham Mission was also based in Algiers at the same time, but with a different area of responsibility.
Secret Flotillas: Vol. II. Brooks Richards
The Small Scale Raiding Force. Brian Lett