In reply to Rick Graham:
I always buy it - only used it once. Snowy school drive and I was stationary and woman skidded into me - apologised, said she was in a rush to pick her child up and gave me her phone number/details to sort it out later. When I phoned, the husband (who wasn't there at the time) said it was knock for knock.
I handed it over to the insurance/solicitors, got my car fixed and no claims preserved and the other team paid for it. Would have been pretty tricky otherwise,
Chris