Cobnuts come mostly from Kent in the south of England, where there is a Kentish Cobnuts Association to promote them.
I bought mine in Waitrose.
Edinburgh is a long way from Kent.
After a careful whack with a small hammer (because I do not possess nutcrackers), the shells cracked...
... and the nuts were revealed. It's as though each nut has the rings of a tree on the surface.
They were tasty too. Like a sort of milky hazelnut, which is, I suppose, not unexpected.
I think there may be something similar in the garden, time to investigate.