What was the fire originally before you wanting an open fire? if it was a back boiler type surely it was used as an open fire. Try testing with a smoke bomb from a garden centre if the smoke goes up the flue fairly strong stream of smoke then I would have thought you could use it without taking out the back plate. The old fireplace back plates are bolted in so you have to take the whole surround out which is a job and half. Another up draught test is to place a sheet of newspaper [two pages for size] across the front opening and you will see if the up draught is strong the bottom part of the paper drawn in. let the paper go and if it stays or get pulled in then the up draught is there for an open fire without removing the back plate which incidently protects the back wall agains heat.
Nobby, September 2008
