Insulating a timber frame extention

[Candyman85]

Hello folks
I've got a problem with a timber framed single storey extention it's very cold throughout the bathroom bedroom and front door area.the extention was built about 25yrs ago and it has no insulation in the walls and only one small rad.ive been told you can't get normal cavity insulation done on it so I want to know what options I have,it just sucks all the heat out the house and it's not great in the winter in fact it's s**t! any help is really appreciated

 

[^woody^]

The only viable option is to take the plasterboard off and insulate the frame with something like Celotex.