try removing the P.Sw cable from your mobo and shorting the two pins momentarily with a clip or small screwdriver, if this turns it on fine then your switch or wire is at fault, if not then its likely more serious.
Thanks for your messages. It was the PSU in fact... I just switch it off after using the PC and when I switch it back on from the back, my PC starts straight away. Something to do with it decharging I heard... Might need to change it if it starts playing up again.