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.


Literature
1 Nederlands Soortenregister2 Aukema, B., Heijerman Th. & Kalkman V.J., 2016. Veldgids wantsen deel 1. – EIS Kennis centrum Insecten, Leiden.
3 Waarneming.nl