Is the attic 'converted' already ? If not, and you can see all the timber work supporting the roof, it's not too late to fix.
Is the ceiling/floor, floored, yet? If not, insulate it (I had my attic done with Warmcel recently), and floor it - right out to wall plate connection.
Then, looking at the rafters, if some parts of the insulation are particularly bad, remove and replace the offending pieces. Small gaps, etc, can be foamed up to make tight. Pay attention to around the wall plate junction, etc. If the whole thing is very bad, it might be worth just re-doing the whole area.....
I'd then fit a vapour proof layer over the entire surface, taking care to seal well around wall plate detail, etc.
Next I'd fit an insulated plasterboard, again over the entire surface. Again, not too difficult.