Saint Jack
Secondary servo pack on a Wessex...a rigger job, natch. However, on 240OCU the riggers wouldn't change them until the poor insty had changed the associated servomotor in situ. Swine of a job, you needed six inch long fingers with integral wire locking pliers to get it done.
The yaw servomotor was a much easier proposition, it was on the transmission deck and you had good access all round. I only ever had to change one of those, and wouldn't you know it, the a/c had gone u/s in a field in Norfolk. In January. In the snow.
Central suppression unit on a Tornado. The bit they started with: the rest of the aircraft was built up around it. Gazillions of cables and no access.