Concrete Slab Insulation

Hello if I lay polystyrene insulation (25mm) upright against the walls for a concrete floor slab as required by building regs then which height do they go to? Do I lay them to the same height as the concrete then screed over them in level with the concrete? Is it okay to screed directly over upright polystyrene? Won't the screed run down the small gaps at the sides of the polystyrene? I'm confused please someone explain this to me thank you very much

The reason for the 25mm insulation upstand is to stop thermal bridging between the inner skin of the walls and the slab / screed. So I would have said that the screed should also finish flush with the top of the insulation.

Thanks Eng33 for the excellent quick reply makes perfect sense