Since April 2002 Chimney Lining comes under building regulations. Many existing liners were done before then, by untrained engineers on a DIY basis.
Any existing chimney, new flue or chimney installation must be given a visual inspection to check that it is in good order. The flue should be clear of obstructions and is of a suitable size and type for the appliance installed.
It may be necessary to sweep the flue and to check it for gas tightness. This must be done before fitting a new stove or boiler.
As of October 1st 2010, a CO2 Alarm became a legal requirement. Extra regulations were also added dictating exactly what size outlet and flue diameter are necessary for a given appliance.
Not only will the stack needs cleaning and inspection, chimney pots will need removal. If the new liner is larger than the existing chimney pot, then new pots will be needed. If the liner smaller, then the existing pots can be carefully removed and re-mortared into place again.
Lots of dust may be generated whilst a stack is being swept or lined therefore it is essential that adequate precautions are taken before hand.
This is a job best left to the professionals.