Yeah Davids right. It would have been the bloke at the backs fault, as he either couldnt have been paying enough attention or was too close to car in front. He's also right about you claiming off guy in middle and then him claiming from guy at back. Thats what happened with me, the guy in from claimed off me, and my insurance company just passed it onto bloke at back.
I'm surprised that not even the guy in middle got any details from the guy at the back, as there must have been some damage on his if he was shunted forward.
Unfortunetely you're just going to have to put it down to lesson learnt, as if you didnt get ANY info, like a phone number or reg plate then there is nothing you can do other than pay for any damage yourself.
I couple of weeks ago my missus was going round one of those piddly pointless little roundabouts when someone pulled out on her so she had to stop suddenly and the car behind her hit her. Was only going smail pace, infact my missus didnt even know it happened untill the girl behind come up and knocked on the window. She didnt get any details either as she though everything was OK. Fortunetely there was no damage, but I still had a right go at for not getting any info

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I'd go out and check it now, if you're allowed, just check underneath and lift the mat up at bottom of boot and see if there is any buckling by the spare wheel well.