Existing chimneys & flexible chimney liner

Lining existing chimneys - in Document J, page 20,29,44,64

Lining existing chimneys

Flexible chimney liner may only be used to reline existing chimneys. The chimney liner must be suitable for the application and the manufacturer's fitting instructions must be followed.
Chimney liner must be installed in one continuous length with no joints within the chimney.
Do not use coring balls on any metal flue system.
We would recommend using appropriate sweeping brushes to ensure that the chimney is not blocked by chimney deposits.

Insulating the liner is not specifically mentioned in Doc J but is a very good idea.