I have marble sized hard lump that feels like bone in the centre of the back of my skull, in line with the top of my ears. It seams to be symmetrical.

Boney prominence. It probably is the occiput, a boney prominence in the center, back part of your skull. Not likely of concern, but you should see your doctor to check it and make sure she/he does not suspect any other problem like a scalp lesion, cyst or boney growth.