Calculations concerning the competition between radiative and non-radiative decay for doubly-excited 4lnl' (n=4, 5) singlet states in Ne8+ are reported. Recent coincidence experiments have been interpreted as showing that for the 4l4l' states in Ne8+, as well as in the isoelectronic N5+ and O6+ systems, radiation contributes more than 50% to the total decay. The 4l4l' states in Ne8+ are, as in N5+ and O6+, degenerate with the 3lnl' Rydberg states and a considerable mixing exists between the two, which leads to an increase in the radiative decay probability. In the free Ne8+ ion we find that radiative decay accounts for 29% of the total decay for the singlet terms assuming a statistical population distribution. The results vary considerably between terms and the total radiative yield is strongly dependent on the initial population distribution. The branching ratio is similar to the ratio obtained under similar conditions for O6+. The similarity is ascribed to two competing effects: the lowering of the 4l4l' states relative to the 3lnl' series with increasing Z, which favours decay through autoionization, and the Z4 behaviour of the radiative decay rates, which has the opposite effect. The free ion results are compared to results including the auto-transfer to Rydberg states (ATR) mechanism. In N5+ we have shown that this effect leads to an appreciable reduction in the radiative yield. For Ne8+, the ATR mechanism is less important but we conclude that also here the effect on average is a (small) reduction in radiative yield. This is because the ATR mechanism is usually so effective that the 4l4l' population is transferred to low Rydberg states. These autoionize in light ions while they have a larger probability for radiative decay with increasing Z. This seems to account for a large part of the difference between the ATR results in N5+ and Ne8+. We report the results of an alternative approach in which part of the collision dynamics is included and we find that the ATR model is rather sensitive to a number of parameters, which at the moment are not known very well. For the 4l5l' states, which are located above the N=3 threshold, radiative decay accounts for 6% of the total decay, much less than for the 4l4l' states, due to the opening of autoionization channels to the N=3 thresholds for the 4l5l' states. Assuming a statistical population distribution, the theoretical results are in clear disagreement with recent observations for both 4l4l' and 4l5l' manifolds. However, we conclude that it is necessary to obtain a more definite idea about the real population distribution before it is possible to make meaningful comparisons with the observed radiative yields.