Lawson et al. (2009a) gave details of a comparison between measured and modelled or theoretical C II to C IV line intensity ratios of emission from the edge of the JET plasma. The radiation was recorded with two spectrometers that observe the VUV and XUV emission from the plasma along a horizontal line-of-sight. Line intensity ratios were calculated using a collisional-radiative model and discrepancies were found with the measured values. However, for the VUV instrument a very accurate sensitivity calibration was only available at shorter wavelengths. At longer wavelengths the accuracy of the sensitivity calibration was poorer, limiting the usefulness of some of the measurements. In addition, the paper investigated various possible causes of the observed discrepancies, but no satisfactory explanation was found. In the present paper, the discrepancies found for the C IV ionization stage measured with a VUV spectrometer having a vertical line-of-sight that views the Scrape-off-Layer (SOL) above the throat of the outer divertor are detailed. This spectrometer, which has a wavelength range of 140Å to 443Å, is ideal for observing the C IV ionization stage and has the advantage of an accurate sensitivity calibration throughout its wavelength range. It is noted that measurements with this instrument of the emission from the divertor plasma show agreement with theory to within the experimental accuracy (Lawson et al., 2010). Two further possible explanations of the discrepancy are investigated here. These are differences resulting from the line-of-sight averaging of the measurements across a plasma region with steep gradients and the possibility of non-Maxwellian electron energy distributions affecting the electron collisional excitation rates. The results of these further studies are presented.