From my bumper book of building regs


"The circumstances in which a flexible flue liner may be used in a
chimney are if:
- The liner is in accordance with BS 715: 1989 AND
- The chimney was built before 1 feb 1966 or is already lined or
constructed of flue blocks in accordance with teh apprived document"


Under certain circumstances if the chimney is lined with appropaite flue
blocks a liner is not necessary. I THINK.

You can get double insulated steel flue sections for later chimneys.
These are expensibve.

Inserting them can be even worse if the flue is not straight up and down.

Dodgy stacks and brickwork MUST be rectified befiore lining as it
dangerous to work around crumbling stacks.

Proper H & S adherence means usually the minumum of some scaffold
erection, or working off an elevated platform. Soem self emplyed guys
may rowrk of allders and use straps, but no self respecting company with
liability insurance dares take the risk.

Which makes teh cost anything from 200 to 2k to line a flue...