I haven't been able to identify these, but they could be of the genus Cerceris, called knoopwespen in Dutch. The ones in the photos were very small, about 10mm (0.4").
Very big eyes on this one, well, relative to its body, that is:
Olympus Stylus 1s, 42mm, ISO800, f8, 1/20s
A similar one, digging away:
Olympus XZ-1, 28mm, ISO400, f4, 1/250s
All hints for identifying these are welcome, but it's fine if you just look at them as well.
