An understanding of the occurrence of opacity is important for modelling the power exhaust of magnetic confinement fusion plasmas. The absorption and reemission of radiation alters the magnitude and distribution of radiation within the divertor of the fusion machines, the plasma volume within the divertor controlling and limiting the power and particle fluxes to the plasma facing surfaces. High levels of opacity can also result in changes to the ionisation balance within the plasma. An analysis of a discrepancy between He II (He+) line intensity measurements observed on the JET machine and the collisional-radiative models describing them has resulted in a better understanding of the behaviour of the radiation from He II. This has led to a simple technique for monitoring opacity within the He II ionisation stage. The technique has enabled three periods of He operations to be surveyed in order to determine the occurrence and severity of opacity in He discharges. A precise calculation of opacity is difficult, although estimates of escape factors and optical depths are possible for all pulses in which opacity is judged to be significant. These estimates do not fit the available line-of-sight opacity models and allow an alternative model that more closely matches the experimental results to be derived. The sensitivity of this model to its various components is discussed.