Barclay was a passenger on Bus 200 when it travelled through a wormhole. He was going to Tina's, who, as he commented to the Tenth Doctor, was not yet his girlfriend. He did a two-week NVQ on mechanics at a local garage, and this contributed to his knowledge of mechanics, despite not finishing the course.
When he was returned safely home, the Doctor recommended him to Captain Erisa Magambo for a position in UNIT because he was good with engines. (TV: Planet of the Dead)