Some time ago there was a thread about roof ventilators. Our house has been very hot in the living room/kitchen, in fact It was hotter inside than outside, so I decided to get a couple of the roof ventilators fitted. Do they work? Yes its much cooler in the house, there is no ventilation from the living room to the loft, but it still seems to work, I don't want to fit ceiling vents as that would make the air con work overtime.
Yes you will notice a difference since you're allowing a large volume of hot air that was previously trapped in your loft to escape. You might want to add some air inlets to the soffits to ensure free airflow in the loft void in order to prevent the build-up of condensation and mould.
Fortunately I did check that before they were fitted, the soffits have slotted holes all the way round the house. I really need to get up in the loft to have a look round and see how hot it gets, The roofing sheets have aluminium foil and some sort of plastic insulator on them.