Harry, they appear to be an intentional variation on smooth spheres see images http://en.wikipedia.org/wiki/Carved_Stone_Balls mostly with 6 knobs each.
Megalith construction aid A theory on the movement of 'monument stones' has been put forward as a result of an observed correlation between standing stone circles in Aberdeenshire, Scotland and a concentration of carved stone balls, and it is suggested that these petrospheres may have been used to help transport the big stones by functioning like ball bearings. It has already been established that many of the late Neolithic stone balls had a diameter within a millimetre of each other, which indicates that they could be used collectively in some way rather than individually. By plotting the find sites on a map it can be demonstrated that often these petrospheres were located in the vicinity of Neolithic recumbent stone circles. Models using small wooden balls placed in a groove in parallel longitudinal pieces of wood 'sleepers' with a carrying board above have shown such megalith transport to be practical in some situations
