OK Wanty ........... I guess what you have is "dry-lining", with vertical "battens" fixed to a conc. block wall, then plasterboard and plaster.
Also, it sounds like it's an exterior wall.
If I'm right, it's a very common problem......... comes up on here all the time.
You badly need insulation fitted between the battens (Celotex or Kingspan board or similar.) If 25mm thick battens.........then 25mm insulation. If 50mm battens, then 50mm insulation........ etc etc ............ the thicker the better.
The insulation is just cut to fit snugly between the battens, then it's all plasterboarded and skimmed.
If there's a roofspace above the room, then make sure there is plenty of insulation around the junction of the wall and the ceiling (that's the thermal weak spot.)
Ventilation is essential. If your windows don't have "trickle" vents to give a little background ventilation, then you might want to consider one or two discrete small grills in the ceiling........... vented into the roofspace.