Insulation for outside wall

[net64]

Hi hope someone can help me.I have a playroom /den which was a garage space which we got converted into a room before we moved in.
When the heating is on it is lovely and toasty but when it goes off it gets cold very quick and is like an icebox!
There is insulation in the attic above(it has a tiled roof with no bedrooms above it).
I was looking into getting the cavity wall insulation on the outside wall and was wondering if anyone on this site has any views on it?
Also is it worthwhile putting extra insulation in the small attic above?
Can a wood burning stove be put in seeing as the roof is only half the height of the house roof?
Any thoughts welcome.

 

[PADDYBOY99]

How thick is the insulation in the attic?
The insulated slab( interior) may be easier for you rather then the outter slation.

 

[net64]

Not sure how thick but it fills the rafters and was put in when house was built so I presume is the standard required.
I just think that because it is an outside wall that the heat doesnt stay very well and was thinking of putting extra insulation in the attic and insulating the external wall with cavity insulation.
There is insulation behind the plaster board in the external wall but was just wondering had anyone on the site have the cavity insulation . On a day like today when it is snowing it is freeezzzziiinngg when the heating is off!