Flock constrain to surface causes weird shadows - help

[cookey]

I created a landscape using the terrain wizard and wanted to populate it with flowers. I decided to use the crowd flock and tested it with 16patch spheres 1st from the primitives library. I did a quick test render and sure enough there were the spheres floating just above my landscape. I then used the "flock to surface constraint" to constrain the spheres to the landscape surface. Sure enough, after scrolling through the frames a bit, the spheres moved onto the surface in my choreography window. When I did a test render, much to my confusion, some horrible great big black shadow things had appeared on the landscape, as though some massive thing was casting a shadow. I've only got one light in the cho, just a simple bulb and it does the same thing in all the versions I have (13,14,15). As soon as I remove the flock to surface constaint the rogue shadows vanish.

 

If anyone knows what is causing this and how to fix it then I would be most grateful. I've attached two images, one showing the scene without the constaint and the other with it. Cheers.

 

Also it seems that the objects in the crowd do not cast shadows onto other non flock objects, is there a setting somewhere to make flocks cast shadows?