You will need building regulations approval if you are replacing the appliance, hearth or chimney (or installing a new one) - if in doubt speak to your building control surveyor and consult Approved Document J which covers the installation and replacement of heat producing appliances like boilers and stoves.
This gives guidance on all aspects like efficiency of new and replacement boilers, the position, specification and installation of chimneys, flues and hearths, and offers advice on safe fuel storage installations, including solid fuel, liquid oil fuels, and gas-fired heating.
It outlines the procedures and specifications for the adequate provision of combustion air, the detection of dangerous carbon monoxide gas and suitable warning systems like detectors and alarms, protection against pollution and the control and ventilation of combustion by-products like smoke and fumes.
The only exception to fitting an energy efficient condensing type of boiler is when your approved installer considers that the position of the condensing boiler is exceptionally problematic. Your installer should be able to advise you about these requirements.
Heat producing appliances and stoves should be installed, commissioned and certificated by Competent Persons.