A system for servo-control of the plasma position in a tokamak discharge is described. Measurements of the stability boundary of the servo-loop in the Cleo-Tokamak apparatus are presented and compared with a theoretical model. They show reasonable agreement. Using servo-control, the discharge time was extended from 110 ms to nearly 200 ms, and the plasma remained within ±10 mm of the centre of the vacuum vessel for up to 140 ms. The instability caused by a deliberately applied vertical field with positive gradient was stabilized.