Alfie also eats the plants and munches on the stones. I havent been able to stop him so far, I just make it as difficult as I can for him by putting pots infront of the plants he favours.
Ive not noticed him swallow a stone yet, he does carry them around in his mouth for ages, My last cocker also loved the wee shingle stones and never swallowed them either, just munched on them