Hello, thank you all for your response's to my enquiry.
Tried the cat litter, bag weighed 5.2kg and it was very firm.
Our nearest ASDA (ex Walmart) supermarket has 1.75kg bags of dried split lentils, or dried red beans, or dried chick peas.
I liked the feel of the chick peas because they appeared to me to be a little more forgiving than the other two.
So, cat litter out chick peas in and my bench rest bag now weighs 4.1kg.
My air rifles now sit in the vee of the bag rather than on the top of the bag, and appears to me to be a more stable support.