Double plasterboard in attic?

I have a dormer attic, which at the moment is insulated with 100mil roll, covered by a layer of plasterboard which has been very badly installed by builder (not taped, edges loose, nailed, etc). I want to finish this space, and am considering installing a new layer of plasterboard over the first, and skim this. Should I:
a) do this,
b) put a layer of rigid insulation between the plasterboard layers,
c) tear down whats there and go for high spec insulation and one layer of thermalboard?
d) do something else?

thanks,
Delia Q

There is a danger of interstitial condensation if you apply another layor of insulation and board over the existing build up. Your best solution is probably to take down what's already done and start again. Energy costs are going up so now is as good a time as any to tackle this.

Thanks, thats something I was concerned about. I'll tear down and start from scratch.
-DQ