Got mine from ebay seller martyns-bargains, 5kg for £14.99 free shipping. Works perfectly.
Hi Ronnie, guess it must have some sort of effect but I just bought the one I mentioned & it brought stuff up very nicely! Very good price & enough to last ages.....
pets at home do sell walnut shell, you need to look in the cat litter section!
http://www.petsathome.com/shop/en/pets/cat/cat-litter/cat-litter-21103--1
Martynsbargins was the seller I bought from too I can't complain should last you a while Ronnie.
I did tried some jewellers rouge as an additive but is a total pain to remove afterwards, last batch I added a couple of squirt of Novus 2 which worked well.
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