There are 12 different Ancistrocerus species in The Netherlands of which 6 are quite rare.
The other 6 are very difficult (if not impossible) to tell apart from a photo.

Potter wasps are solitary and feed on flower nectar.
Potter wasps make mud nests for their eggs and larvae or use abandoned burrows of other insects. 
They stock the nests with caterpillars and other grubs, which they seal in with mud. 
These are eaten alive by the wasp larvae.
Potter wasps have the potential to deliver painful stings, but are not aggressive and rarely attack humans.

1st photo: In my yard, Hilversum, The Netherlands, 1 September 2012
2nd & 3rd photo: 's Graveland, The Netherlands, 28 August 2012
4th & 5th photo: In my yard, Hilversum, The Netherlands, 3 September 2018