A friend of mine had insulation on the ceiling of his bungalow and his rooms were pretty damn warm. Nothing he did seemed to change matters much even after dusk.
I suspect that the insulation was preventing the warm air created by the sun on walls and glass windows from being released into the loft space.
I'm no expert but maybe the insulation would be better sighted under the roof tiles. Having vents installed at either gable end would additionally ensure proper air flow.
The additonal cost involved may prevent a fair bit of discomfort later in the day.
Just a suggestion.
Nice thread by the way Nelly.