whats wrong with my rover 400 deisel?

Intermittently over the last 6-7months my rover wont start immedialty on the turn of the key i get a single click noise,occassions like this when it persisted a jump off leads would start it, recently this has been happening more often however i wait for a little will then try again and it starts sometimes or needs a kick start from leads. (Ive noticed on occassion a high pitch two tone sound but this dosnt consistently happen) Now its happening all the time (ie wont start on the ignition key). theresJust a single click noise I know the battery is fine cause i changed for a new one recently. Now i have to get a push start or roll down a hill and that turns the engien over and the car runs fine. What might it be?