The control of the plasma shape and position in a tokamak requires a fast, reliable and sufficiently accurate method of determining the plasma boundary. At JET, a method using Taylor series expansions of the magnetic flux surrounding the plasma has been developed which meets these requirements: the calculation takes lms per time slice and comparisons with Langmuir probe and CCD camera data suggest the accuracy is approximately lcm, comparable with the full Grad-Shafranov equilibrium codes in use at JET (EFIT, IDENT). The expansion method has been adapted for use in the pumped divertor phase at JET where it is now used as part of the co,ntrol software for the plasma shape and position system. The plasma boundary data (position and poloidal field) is also supplied to the FAST code to calculate some global parameters such as poloidal beta, internal inductance and plasma energy. Good agreement with calculations based on diamagnetic measurements is obtained. This information could now be used to control the plasma beta or energy and extend the duration of H- and VH-modes. The representation of the plasma boundary within FAST based on the Lao- Hirshman expressions has been extended to give a better description of the asymmetric single null plasmas which are now predominant at JET.