In article , Jaques
d'Alltrades writes

They are going in through a tiny hole next to the mortar in the top
course of stones, about 6 inches below the chimney pots. It seems to be
one arriving or leaving every minute or so, so it doesn't seem like a
large colony. They look to be bumble bee type things, round and fluffy,
with white bums, and rather small for a bumble bee. Any thoughts?

Mason bees perhaps?

If that's what they are, then putting a liner in might disturb them, but
shouldn't ruin their home.

Good :-)

I thought you were talking of a honey-bee nest actually in the flue. If
these are mason bees, they'll be in a cavity in the brickwork.

Stonework not brickwork! Bigger lumps, and bigger opportunities for
cavities

So all I need to do now is convince the builders that they'll not be in
danger? ;-)
--
Kay
"Do not insult the crocodile until you have crossed the river"