I recently had a brick garden wall built so had 3 bricks left out, the idea being to fill them with those special tube things and also some broken pieces of cow parsley stem to see which they preferred.
I can report so far no interest whatsoever, but the wall wasn't finished till latish spring last year, so might have been too late then.
If it changes I will let you know.
The resident solitary bees nest in a south and west facing wall.
All the best
Sue