Solitary bees building nests

I think the bees that nest in my "bee hotel" are red mason bees (Osmia bicornis) but I'm no expert. Several cells in the "hotel" were used last year, and now I have at least three bees building new nest cells.

The bee enters the cell head first, and after 10 or 20 seconds turns round, then another 10 seconds or so leaves head first.