The Doctor places his bat between the middle stump and the one closest to him, called the leg. (COMIC: The Tides of Time)
Middle and leg was a batting position in cricket. The Fifth Doctor once asked an umpire named Bill for a "middle and leg" guard, to which Bill replied that the Doctor should move his bat towards himself to cover both the middle and the leg stumps with his bat. (COMIC: The Tides of Time)
On another occasion — when he was playing for the Stockbridge Cricket Club — the Doctor again called for middle and leg. During the runs that occurred as the result of the Doctor's successful bat, Don's died on the cricket pitch, having just barely scored the winning run. (AUDIO: Autumn)
The Fifth Doctor didn't always bat middle and leg. He appeared to alternate between middle and leg and middle stump guard when playing on Charles Cranleigh's side. (COMIC: Time & Time Again; TV: Black Orchid)