Taking out a gas fire place

[oldexpat]

I have a gas fire place in my dining room which I wish to remove. I can remove all the mantle piece etc but how would I go about fillingf up the hole?

[The Builder]

The simplest way is to fix treated batten (25 x 50) around the inside of the opening. Fix plasterboard to the batten and plaster to finish flush with the existing plaster.
Or, use MDF, plywood or similar and simply caulk the joint .......... again finishing flush.
Then cut an small opening to take a flush vent .......... to keep the flue ventilated.
Your existing gas cowl may be ok to leave in place, or, if not suitable, you may have to fit a rainhood or similar ventilated chimney pot.

[KARL]

If you want to do a permanent removal of the entire fireplace installation then the way to do so is to remove as much of the external chimney structure as is consistent with any other stacks that share it and are to be maintained. Then totally fill the chimney flue with a solid material between the very top and at least down to below the roof line and/or to where some degree of heating is maintained. This can usually be done relatively easily by fitting a plug of some kind below the roof line and then filling the pipe with expanding foam, in lifts/stages of 0.5m or so maximum each of which you allow to set before the next lift. You additionally plug the bottom end of the flue (top of fireplace) with expanding foam. Then use mortar or other appropriate material to cap/seal the very top from the weather. There is then certainly no need for a vent into the room. If the flue is entirely in an exterior wall (e.g. gable) then the entire length of it may need to be filled (depends on materials, etc.).