My new HIFI room

[Barry]

it's 12 foot square and it's been insulated with polystyrene before being plaster boarded, the ceiling is low and acoustically it's dead, I don't know if that's because of the polystyrene insulation

Could be. What is the height and what are your speakers?

It's always best when the three dimensions are dissimilar. In your case two are identical (12'), so you will need to do something to make the left/right (from the listener's point of view) acoustic different from the fore/aft acoustic. Ideally, if given the choice, speakers should occupy the 'live' end of the room, with the listener (sat on his nice comfy sofa), at the 'dead' (or deader) end of the room. Put your record storage racks at the listening end, or hang up some heavy curtains there.