I use Thornton's to purchase gifts, manly at Easter and Christmas time. I never have any trouble with cashback, which is paid within the expected guidelines.
Their website is easy to navigate and they often have freebies on their site for hitting spending limits,eg, free chocolate santa model at Christmas for spending £30.
I did wonder whether these freebies would invalidate the Quidco payment but they never do. Very impressed with Thornton's overall.
My only negative is the amount of packaging used. However, it does protect the chocolate so a bit if a necessary evil.