Here is some info for a rock that must be close to the Slaggyford examples with the same type of markings . Well recorded with clear pics and equally decisive evaluation of "not rock art " .
http://archaeologydataservice.ac.uk/era/section/panel/overview.jsf?eraId=1366
http://archaeologydataservice.ac.uk/era/section/panel/media.jsf?eraId=1366