Fireplace boarding and hearth install

[Grace24]

Hi!

We have just removed the mantle and surround from our fireplace and are left with a good sized recess.
I want to board the recess to be painted and fit a hearth that I can tile over. Best way to go about this as the base of the fireplace is currently higher than the floor as seen in the attached photos.
We will be putting in an electric plug in style heater (one of the ones that looks like a log burner) so shouldn't be any excessive heat.
I'm assuming I need to chisel out the bottom to level it? And then what would be best material for the bottom to put tiles on top of?

 

[blup]

If you do as you suggest and chip out the hearth it wont involve anything structural and all can be levelled up. The socket can be extended but should be fixed against a timber frame designed to take the plasterboard or other covering.