Newbuild house has an electric fireplace. This mounts in a mantel which
is attached directly to an outside wall, but there is no vent and no
chimney.

Kinda like this, but designed by someone sober..
http://tinyurl.com/7dkre

There are no skirting boards on at the moment, so I have access to the
little gap between the plasterboard wall and the floor. Will I be able
to use this gap to route wires behind the mantel, or will it get too hot ??
I assume there will be lots of heatproof gubbins behind the fire to stop
the plasterboard getting messed up...