what to do with hole in wall after gas heater removed

hey guys, i'm fairly new to diy and would like advice on the following situation:
ive just had a gas fire removed and would like to cover up the hole. there is already a metal plate attached and i am thinking of placing an electric fire in front of the hole.
would there be any issues with me just leaving the metal plate as it is and placing an electric fire in front? if not, what would be a better solution?
on a side note, i have issues with mice in the property and would like to fully block any holes in the chimney section, would there be issues with me doing this?

 

If you fully block it up, you could suffer from damp. If you want to block it up we (Company I work for) use thermalite blocks with a purpose made hole in it, plaster it up and a vent fitted (to prevent damp) You could leave it as is with the closure plate also and fit an electric fire.

 

might then go for 2nd option....thanks!