Confidence: It might be this.
Notes: Not sure what this bee in our bee box is, and I hope someone can help me at least narrow it down. I assume it is some kind of mason bee, but doesn't appear to be either osmia rufa or o.caerulscens. The tube is 1cm diameter.
Unfortunately it has yet to emerge when I am around, and indeed retreats further into the tube when I approach. Took some patience to wait until it ventured this far.