I have a loft conversion that is inadequatly insulated (according to the
survey). The current room is platered?

What is the easiest way to inprove this insulation?

I have been looking, at products such as Knauf Linerboard, or X-Foil
(http://www.just-insulation.com)

Could this be attached directly to the plaster, overbatterned and then
plasterboarded? or in the case of Linerboard, installed directly over the
existing plaster?

Would an additional vapour barrier be needed?

Any suggestions?