Thanks for all the advice. A few answers (and a result?)

Yep - the fire was professionally fitted by a company.

I have been told a fire was originally installed. Although it's dubious
why the last owners replaced the fire with an eletric one.

The wall is too hot to touch.

The fire uses natural gas, with injectors I think. Not sure if any of
the previous fires used injectors.

I read the installation book that the fitter left behind last night. It
appears that I have a pre-cast flue, meaning that I've got a load of
concrete blocks with a hole in them going to the loft. Once it's at the
loft, I've got a metal pipe going to the raised roof tile. The plaster
has been applied directly to the concrete block, with no insulation.

According to the installation book, a new standard BS EN 1858 (i think)
published in 2003 says pre-cast flues must have insulation between the
concrete and plaster. It also says if it doesn't have this insulation,
no gas fire is suitable for the flue. (does this mean no gas fire with
injectors?). Looks like I'll have to opt for a balanced flue fire, or a
electric fire.

As the installer fitted the wrong type of fire for our house (although
it would be difficult to tell if the plaster is applied directly to the
concrete during the survey), I imagine I'd be able to "do a deal" on
the replacement fire.

Anyone know if it's common for pre-2000 houses to have pre-cast flues
with plaster stuck directly to them?