Building regulations are valid for 3 years after which you will need to re-apply under the current regulations in force at the time. Therefore, your property should have been installed with 270mm insulation.
You will need to contact the local authority regarding the planning permission as it may fall within your permitted development rights.
Assuming it has been adequately supported, to save money probably best leaving the chimney as it is.
You will need to obtain building regulations for your...