Pat could be right Euan. I used to buy some on line they'd happily eat them for months, then suddenly didn't bother. Then they'd be interested again.
Home Bargains sell two types, being me, I bought the cheapest ones which they devoured for two years, sometimes I couldn't keep up with them, 4 in the morning, then another 4 in the afternoon!! Suddenly they wouldn't touch them. The same scenario at our edge of town feeding station. So I switched to the dearer ones and they seem happy with them.