i've had a new fire put in (class 2 flue), and the fitter told us to expect
a very hot chimney. It seems that when the house was built, the builders
didn't install any reflective plaster board on the chimney, and also put
down very thin layer of plaster.
....and the fitter was right. It is very hot

Is there anything i can do, aside from the large task of installing
reflective plaster board on the chimney - re-platering - getting fire
re-fitted, etc. Is there any special paint i can put on, as a sort of
undercoat to keep the heat in the chimney??

also, what are the consequence of doing nothing?? Peeling paint perhaps??