Insulating/decorating a summer house

I have a 5M x 3M summer house in my garden which i am thinking of insulating so that we can use it more often.
It is basically 100mm wooden battens with tongue & groove outer skin.
I was thinking of insulating between the battens with 70mm Celotex or Kingspan or similar and then boarding over the top with moisture resistant plasterboard and which i can then fill and smooth the joints myself e.g:
http://www.insulationexpress.co.uk/...-Lafarge-Moisturecheck-Board-Tapered-Edge.htm
Any comments on this soultion before i get going would be welcomed.

check out kingspan's website for the tech ...also consider breather membrane etc etc
Internal walls....have you thought about 12mm moisture resistant mdf?....easy to use ...tougher than plasterboard....no skim great surface ready for paint

I thought that the tapered edge plasterboard would be better as the cracks could be filled and sanded. With MDF you would be able to see the joins unless you line it with paper or something.
It is a good size space so want to make it as much like a normal room as possible i.e. although it is a shed i don't want it to look like one so much from the inside.
My worry with taped and filled plaster board is that movement in the structure may cause cracks on the joins.
Also if i use Celotex or similar between the battons and foil tape across all of the batton gaps i'm presuming that this would create an adequate vapour barier. Is this right or would i be better off sheeting it with PVC before i board it?