Can you believe it? This was Advantage car hire (booked through Carjet). I have an annual insurance for car rental so I don't need another from the car hire company.
I'm quite good at saying "no" , then "no" again, This was when the booking clerk explained that if I take the insurance I'd be able to take the car back with no inspection and no delay. However if I didn't take the insurance they would have to thoroughly inspect the car which might delay me so much that I might well miss my plane for which they would not be responsible. She explained that for example a problem with the clutch would be my responsibility and implied that they might have to strip the car down to examine the wear on the clutch plates.
This is sharp practice and typical of what is going on now with some of the car rental companies. The staff are clearly on commission to sell insurance by whatever means they can.
The car was great but I won't be using Advantage or Car Jet again.