Sustained operation of high-performance, ITER-baseline scenario plasmas at the high levels of input power (_~< 40MA) required to achieve ~ 15 MW of D-T fusion power in JET-ILW requires careful optimisation of the fuelling to avoid an unacceptable disruption rate due to excessive radiation, primarily from W impurities, which are sputtered by edge-localised modes (ELMs) from the divertor targets. By using a train of ELM-pacing pellets from a high-frequency pellet injector to promote regular ELMs, which  flush W and other impurities from the confined plasma, such high-performance plasmas can be sustained (for ~ 5 s) while maintaining a high normalised confinement factor H_98,y2 ~ 1, which would otherwise be degraded by reducing the pedestal confinement were a higher rate of D₂ gas fuelling used instead of the pellets to mitigate the W contamination. The causes underlying the improved performance and energy confinement obtained using this combined, gas and pellet fuelling scheme is investigated here in some detail.