Quick photo of the diff draining:

There’s an interesting discussion taking place about all this here.

I’ve had a good look at the inside of the differential and noticed that one of the blades on the wheel that passes through the sensor looks a bit bent, which would explain how it easily clipped the speed sensor.

In the photo below, the blade marked with a red arrow is bent left relative to the rest of the blades. This might not look obvious as the camera appears to be square to the blade’s end, but note the left edge of all the other blades can be seen.

In this photo below I’ve rotated the wheel so that the suspect blade is at the bottom – this shows the difference in angle more clearly.

I’ve ordered a new speed sensor and I’ll bend that spoke back with a pair of mole grips. The next question is whether to just refit it and hope for the best, or hold out and have the differential’s bearings replaced. I’ve received one quote of £300 + VAT which sounds a bit steep, I’m waiting to hear back on another.