Sure of the species? They may just have been Mason Bees a solitary species though several individuals may try and nest together. The adults will be dead now and the next generation will hatch from their little burrows in the mortar next spring.

They might not be in the loft - when bees swarm, its because a new queen has left the nest, so half the bees in the previous nest go with her, form a swarm and usually gather somewhere, often on a house wall or similar, before they either take off elsewhere, or find an entry point and start to form a new hive. See if you have a local beekeeper you can ring for advice. I've seen this twice where I live - the first time, the bees swarmed onto the wall and entered one of the flats through the ventilation shaft in the kitchen. The second year, they swarmed again, and this time just gathered in the same area on the outside of the wall, and a beekeeper came and took them away.