In article , Jaques
d'Alltrades writes

You could block the hole in the mortar with something like plasticene
the evening before work is to begin. If they don't emerge from the
chimneypot in the morning, that should fix the problem, and any worries
about the unlikely possibility of igniting the nest.

And then let them out again when the work was over? Do you think they'd
be OK? (Apart from cross, that is)
--
Kay
"Do not insult the crocodile until you have crossed the river"