We noticed that the grass in our field was looking a bit scrubby, and the earth was being scraped over. At first, we suspected our dog, Watson, was looking for moles, but the damage was becoming too widespread.
One of our neighbours solved the mystery for us – we have a plague of Cockchafer larvae, known as Engerlingen in Dutch. Apparently, we’re not the only ones around here, it’s becoming a common problem in this area.
There are no pesticides approved to deal with it, so we’ve reverted to the Mediaeval method of dealing with the Cockchafer – killing the adult beetles or their larvae. I reckon that there are about 50 larvae per square meter, and most of the field seems to be affected.
We’ve been stripping off the turf to expose the larvae and collecting them in buckets for killing and disposal. It’s a very tedious job.
I can think of more pleasant ways to spend a day…
Geoff Coupe's Blog 
Over here on this side of the pond we had pests that look exactly like you pictured. A biological treatment (I think using Bacillus thuringiensis) was effective for us and we have not had the grub problem in years since.
Thanks, Ludwig, I’ll keep an eye out for whether biological treatments are availbe here…