tut tut and still no one mentions approved document L1b of the building regulations -
Planning Portal - Approved Document L
I refer you to the above document, particularly section 5, paragraph 5.8
You can of course, choose to ignore the uk building regulations and any advice contained therein, or in fact any advice given on this forum, but for the want of solid walls - batten it, insulate it, vapour barrier... then board the whole thing with 3/4 ply then duplex (foil backed *) board (* not really needed if youve done the vapour barrier thing but better safe than sorry, specially with the ply) and finally, skim it.
or insulate externally...
or just render and skim it... but youll find batten, insualtion and board a heck of a lot easier..
and warmer.... especially with gas prices what they are...
and condensation (black mould) proof...
and you can hang a 600mm x 1800mm double rad, double convector on it no worries at all...
coving is easy btw...
back in yer box chris.... :rolleyes)