I may of course be missing something but, in what way are they not standard Spidge?
If it's just the Marble looking stone I've seen a few CWGC markers in other than the usual material.
Good stuff on GWF:
Headstones On British Cemeteries - Great War Forum
Could these be in the newer 'Botticino' stone? A search under 'Botticino CWGC' turns up a few references.
Cheers,
Adam.