Insulating a partially-converted loft.

Hi! I'm looking for some advice please. We have a terraced house and the loft was semi-converted. It was insulated below the floor, then a new set of joists and a strong floor put in. The roof side was insulated with silver foil insulation (I don't know the exact sort), a velux window was fitted and then it was plaster-boarded over. All done properly (as far as I know) and signed off by building inspectors, but it only counts as a 'storage room' because the only access is still by ladder. Which is fine for us.

The space is still not very well insulated however. In the recent heat it was almost unbearable up there. In winter it's not bad with a small electric oil radiator.

So I'd like to improve the insulation to make the space more usable in summer. I can think of a few options:

1. Take the boards down and add another layer of the silver foil. I'm not sure if this would cause moisture issues.

2. Take the boards down and add some rock wool insulation between the foil and the boards.

3. Take the boards down and replace with insulated boards such as celotex.

Could anyone advise me please, as a beginner which option of those or any others would be best, and any pitfalls I should be aware of?

Thanks!