Each are dark grey about 10mm long are clay-like and brittle and were found in the bottom of a door jamb - which doesn' t get used very often, there seems to be a white dried case in each cell - (sorry a bit out of focus in left foreground of pic). Looked everywhere for answers, but can find nothing that fits.
Apologies for this late response - this one slipped through unnoticed.
Yes, these are the cells of a mason bee and probably the Red Mason bee, Osmia rufa. They make these cells of mud in crevasses and holes, each being provisioned with a mix of pollen and nectar and a single egg laid in each cell. They often make these cells in unusual places (to us at least). I have seen these in door locks and the gaps in window frames.