Hiya all,
Finally getting sorted after very little sleep over the past x days. Firstly Lewis was awesome, lots of bog trotting and lovely lovely circles.
Now, I agree they are cairns of a sort, I went up to the hill to the east with Margaret and Ron on Saturday to have a look at another one. Its seemingly not as well preserved as the other one.
For me, they draw to mind the small late cumbria circles E of the M6. Those have got cairns inside, and as Moth says, Burl lists those in his field guide.