I've just seen these in store at Tesco Burgess Hill. Hopefully nationwide. I took the photo after I bought the last six bags of blue slate chippings.
Doesn't have a weight on the bag but this is labelled as a 'large bag'. I measured it for reference and the bag is approximately 35cm wide and 50cm in length.
Also white cotswold stone available for the same price.
£4 each or 3 for £10.