"The Natural Philosopher" wrote in message
...

a crap chimney so it'll need to be lined. Will we need some kind of
airbrick anywhere and where would be best to put it if we do?

You will need some form of ventilation. The nearer to the fire it is the
less draughts there well be. If you have a ventilated underfloor space
consider a grille in the hearth going onto it. This is mandatory from
building control point of view. Its size can be ascertained by looking
into the relevant building regulations.


I read the OP's question as does the *chimney* require ventilation for
drying purposes not the room for combustion purposes.
The chimney will be sealed with plates top and bottom but there will be a
source of heat that may be enough to force dry it.

--
Mike W