Providing seating at work?

At my work they dont provide a good chair, its a 2ft stool and the computer is around 1 1/2 meters off the ground. I work in a store but i still have to use the computer to complete rotas payrolls etc.Ive been gettin neck and back problems recently and i am postitive it is due to the improper seating. Ive asked them for a new chair but they have never gotten back to me. I thought there was some sort or law or act that when working at a computer that the employer should provide correct seating? Could anyone tell me this law or act ?