Then courtship became more intense, both snails everting their penises and aligned their bodies toward each other, exposing their right sides to each other. Finally, after a couple of hours, they were sleeping in their copulating embrace, and their eyes retracted. I missed the darting and I couldn't see any protruding darts. Here is a slide show.
Every now and then, I checked on the snails. They slept on the frame of the conservatory most of the day (at 16:45 they were still mating). I just checked (21:40), and only one remains. The duration of mating in this species is about 7 hours.
I have uploaded a video with an early and a later sequecing of the courtship.
No matter how long you've been acquainted with a particular species, there is always something surprising and wonderful you might still witness.