Thanks for your friendly comments. In spite of having slow worms in our garden for decades I had never seen this behaviour before either, so I was somewhat dumbstruck when I saw this (again apologies for the shaky photo):
It was at this point that I returned to my computer to find out what was going on: was this a death grip? Fortunately I found this pretty quickly:
BBC - h2g2 - The Slow-worm
which states
Mating occurs in late April or May, and there is often rivalry between males for a particular female. Some old males bear permanent battle-scars on their bodies. The successful male grabs a female by the neck during a long mating ritual.
After mating, the female incubates the fertilised eggs inside her body. Each young slow-worm develops inside an egg sac, which breaks when the mother gives birth in August/September. During the summer she lies in the sun to help the developing eggs grow. This way of looking after the eggs is called ovo-viviparous, because the young emerge from their mother inside their egg sac.
Let us hope we get some sunshine!
PS to Meta: My bat detector is a Ciel CBD101. Same performance as yours I would imagine