Hymenopteran nest? But which one?

Hymenopteran nest? But which one?

Observation - Hymenopteran nest? But which one? - UK and Ireland. Description: What created this exquisite tiny clay capsule? About a cubic centimetre, attached to a grass stalk about 30 cm above the ground, directly over a small mound of excavated soil (soil is heavy clay), like a mini ant's nest, in the mini meadow in my North L

What created this exquisite tiny clay capsule? About a cubic centimetre, attached to a grass stalk about 30 cm above the ground, directly over a small mound of excavated soil (soil is heavy clay), like a mini ant's nest, in the mini meadow in my North London garden.