Why you shouldn't just seal it solid
Completely sealing a disused flue traps moisture inside the chimney breast, which can lead to damp and condensation problems — a proper cap keeps rain out while still allowing airflow.
Types of capping
Options range from a simple half-round cap over the pot to a full ventilated cowl, depending on whether you might want to bring the fireplace back into use in future.
Doing it properly
A cap fitted without adequate ventilation is one of the most common causes of chimney damp we're called out to diagnose in Bristol properties, so getting the balance right matters.