I provided all the necessary security information but it was like they were just carrying on with question after question to find the one, (an inappropriate one), that I was not sure about, ie the year I registered the card, which was so long ago I could not answer correctly.
They then said, finally, that I had failed their security questions. They also failed to send me details of their complaint procedure, which I requested. It appears to me Sainsbury's do want their customers claiming their cash back!