Effectiveness of Celotex/Kingspan insulation on rafters?

I have a largish loft room (approx 8m * 5m * 3m at roof apex) as part of the house extension, which is partially above the garage. The previous owners have installed white polystyrene insulation of unknown thickness between the rafters and then plasterboarded and skimmed over this.
In the summer the room gets very hot, and in winter very cold. I've been told I have two options:
1. Tear down existing boards and polystyrene and install Celotex/Kingspan type insulation, then re-dryline and skim.
2. Install insulated plasterboard on top of what's already there, and skim.
I would rather go for option 2 as it would be quicker and cheaper, but is it going to be sufficient to insulate the room properly, and what thickness of insulated plasterboard would be best?
TIA
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A further advantage of Option 1 is that you can install a reflective layer between the tiles and the insulation. This vastly helps in the "hot in summer" stakes. It will also result in a thinner result for the same U-value. It is also possible to ensure that correct ventilation has been installed. Someone may have stuffed the entire cavity, leading to rotting problems in the rafters.
Christian.
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