Flat roof insulation

My 60's built house has flat roofs front and back covering just under half the space above the 4 upstairs rooms. The joists which I think are 4 inches have plasterboard below, a sheet of polythene above the joists on which sits a very thin piece of insulation then the wood and felt on top, but nothing between the joists, so must lose a lot of heat.

Previously I have insulated above two of the rooms by pushing 100mm rockwall down between the joists from the loft with a broom. I'm told that is not a good idea as it could cause condensation and rot, though if I left a 50mm gap above that would be okay and I guess I could achieve that with thinner rockwall slabs or polystyrene or foil roll.

What do people advise ? If I have to take out what I've already done, that could be very difficult to remove.

You don't need an air gap.

The insulation will provide a warm surface and prevent air reaching the cold deck to condense.

Ideally there would be a "vapour check" ie an impermeable barrier (such as polythene) just above the plasterboard to stop any air getting into the ceiling void. But as you don't have this, if you just ensure that there are no large gaps in the insulation, and all the void is insulated then it will be OK