I do remember in the early days of NPAS planning being briefed that this sort of situation was 'planned in'. Sounds odd at first, but what they've had to go with due to funding (or lack of it ) is a rigid structure of pilot numbers and soak up the fact that at times when sickness occurs, the next nearest aircraft will attend (even if it's not on shift yet in the case of planned early morning ops). We all know in practice that's a system with very thin cover for coping with unforeseen pilot shortage, and I know personally that during the planning of NPAS, those at the top wanted more back-up built in , it just couldn't be afforded.