and synchrotron photons. The inputs to [synch]{} are an electron energy spectrum, the geometry of the source region (we modelled the lobes as a cylinder with length (incorporating both lobes) 280 arcsec and radius 35 arcsec) and synchrotron flux density measurements (we used the 178-MHz data from the 3CRR catalogue and 1.4-GHz measurements from the image discussed in Section \[observations\]). The electron energy spectrum was a broken power law, with $\delta=2$ at low energies steepening to 3 at high energies, $\gamma_{min}$ to be 100,$\gamma_{max}$ to be $10^8$ and $\gamma_{break}$ to be 6,000. [synch]{} fits the electron spectrum to the radio observations and outputs an inverse-Compton prediction for a given magnetic field strength. The magnetic field strength is found by comparing the observed X-ray emission at 1 keV and the predicted X-ray inverse-Compton emission; we measure a lobe magnetic field strength of $0.175 \pm 0.010$ nT, significantly below the equipartition value of 0.52 nT. Assuming that only the relativistic electrons and the magnetic field contribute to the lobe pressure (@cbhw04 have tested this assumption) then the electron spectrum and magnetic field strength give us a total energy density and internal pressure in the lobes. We found the internal pressure to be 