I'm fairly sure this is a plasterer bee, Colletes sp.
That is one genus in the Colletidae, commonly called plasterer bees or polyester bees because they build cells in underground nests that are lined with a polyester secretion, giving an almost pearly lustre. That is visible in my photos.
Just two of the 54 genera occur in Europe - Colletes and Hylaeus. The latter looks different and normally nests above ground, hence I consider the genus of 'my' species to be Colletes. Also, the whitish banding on the abdomen is fairly characteristic of Colletes.
This photo of C. daviesanus shows the nesting holes, including the pearly lining - very similar to my photos -