Does the SAS recruit MI5 agents?

No, the SAS Regt does not recruit MI5 "agents" (NB an "agent" is a person working covertly for an intelligence organisation - eg informers, couriers etc - NOT a member of the organisation; members of MI5 or MI6 - or any other intelligence agency, eg CIA, Mossad etc - are called "officers"), but it may work with them on operations - eg a local person recruited by MI5 or MI6 to provide knowledge of an area where operations are to be conducted - a guide or informer.

MI5 and MI6 officers and SAS troops often work together (eg MI5 surveillance officers might brief SAS troops on targets, or SAS soldiers may escort intelligence officers operating in high threat environments), and some SAS may be attached to MI5/ MI6 for particular missions. There are persistent rumours (fairly well founded) that the Secret Intelligence Service (MI6) maintains a specialist covert direct action section called "The Increment" comprising selected members of British Special Forces (SAS, SBS, and SRR), but officially MI6 does not engage in such activities.

Additionally, a few former members of Special Forces (SAS, SBS, Special Reconnaissance Regt etc) do join MI5/ MI6 when they've left the military: usually because they possess particular skills (eg close target reconnaissance; covert surveillance etc), and will join the appropriate sections of MI5/ MI6 where such capabilities can be utilised - eg "The Watcher" section of MI5. Others may be employed as trainers/ mentors of prospective field operatives teaching such things as unarmed combat, close quarter battle, survival, evasion, resistance to interrogation, and escape techniques.