Covering up a fireplace.

Hi,

I'm after some advice please.

I have removed an inset gas fire and intend to replace it with a fireplace and electric fire that fits to a flat wall.

How can I block the hole up?

Theres only a single thickness of brick to the back and I was just going to put a piece of plasterboard flush to the wall as it won't be seen, however I started to think of possible damp issues in the future.

Would it be O.K. just to do this?

Even thinking of now usuing some tile backer board or something similar that would cope better with dampness, would this be better than plasterboard and would it make any difference?

Any advice would be appreciated.