Hi, I would hesitate to say that they are necessarily human, but remember you can't rely on size - babies and children end up in shallow graves too. You have to NOT disturb the scene and inform the police. they will arrange for SOCO and coroner's officer to be informed and will deal accordingly. A lot of bones get turned up by people digging in their gardens every year; many are not human and of, they those that are, they are often determined by a pathologist to be "BEYOND LIVING MEMORY". I have quite a bit of experience in this kind of thing (15 years in scenes of crime). The assessment of the bones has to be done by a professional, and there could be important archaeological evidence associated with the burial site. I would urge you to leave it to the experts and call the Police, show them the site and let them sort it. Good luck.