We have opened up to reveal fireplace in main room - not closed off (can see all way up chimney). However we do not want to put a new fire in, just put an old victorian fireback and fire surround in for decorative purposes. What should we do to the existing structure in order to do this, if anything?
If it's only for decoration purposes and you are not intending to install a gas fire or burn anything there - the best way is to seal it off with a metal plate, just on the top of the fireplace opening. This will act as an insulation, stopping heat from escaping through open chimney - you will spend less on your electricity bill. It will also stop dirt falling into your decorative fire place from the walls of the chimney.
Go to a fireplace shop - they know what it is and can advise you on installer.