I have been thinking about this and searching and searching.
I think you could have been right all along with Angleshades, I saw an image on a German website which looks remarkably similar to yours, but that is the only one so far.
Is that a white stripe running along the side of the larva or it is just the light bouncing off it?
I will keep thinking, hopefully, someone else will come in and give you the answer
Edit, do you remember seeng any faint diagonal lines?