This burrower bug is present in large numbersin the garden. It probably is the Forget-me-not shieldbug (Sehirus luctuosus) ^[1].

This species cannot be differentiated from Sehirus morio, based on photographic material, but only by comparing sizes; S. luctuosus is smaller ^[2].
S. morio does not seem to occur in this part of the Netherlands ^[2,3]. I will measure some of the specimen to be conclusive.

References