whats wrong with my van ?

i have a merc 311 cdi sprinter which is not starting properly / sometimes it starts ok / other times when i turn the key there is nothing at all not even hazards work / when i put leads to it its fine ? if i drive for a while its ok to start again sometimes if not the lights come on dash but all you hear is clicking ? ive had a new battery fitted but problem still there ive had an auto sparky look but no joy with him ?