Before taking the car out yesterday, I performed a very quick fix by jamming some folded cardboard between the clip and the cap. I did this because on close inspection the clip did not contact the front part of the cap very well. To my surprise, no more fluid leak. I just have to now come up with a more permanent solution. The cap is probably slightly distorted, the new rubber seal may be thinner, the top of the resevoir may have degraded.
The MC has been on the car for 16 years without any issues before. The problem only started recently after I flushed the entire brake fluid. Perhaps the cap was damaged, the clip warped - who knows.