Thanks to Grunff who OKed the cement mix for the hole where the fireplace was ripped out (done and looking good), and thanks to all who suggested where to locate galvanised metal sheet.
One further question: Now the fireplace destruction has been filled up and replastered I want to do a "hole-in-the-wall" type fire but am having trouble sourcing a box frame type trim. The fire will not be used except for candles so doesn't need to be very robust. I think it would make a nice feature rather than closing it up with some ugly air vent in it. Local fireplace dealers only supply the metal trim with 3 sides for your traditional fires and don't stock the hole in the wall type with 4 sides. One design place can make a bespoke one in stainless steel for £300. choke. There is no way we could make one, don't have the skills, tools or time. Any suggestions as to where we might be able to source such a thing, preferable with mail order as travel to Great Britain is out of the question at the moment.
thanks as always Suzanne