Seems a bit funny to me.
Lots of overtaking braking etc .... Not a straight forward - "I was going into a roundabout and the guy behind just didn't stop"
Quite simply - the other party is at fault if you were rear ended. You have to be supplied with the name and address of the driver and reg of the car at the scene of a damage only accident. If it's an injury accident, you also have to be provided with the insurance details. If you are now injured - I advise reporting to the police.
There is no reason for you to be out of pocket if you're involved in an accident and it's not your fault - or if it is your fault - it should just cost you your excess unless your just 3rd party - that's why we are all insured.
Make a call via the details on the card - ask to meet and get it sorted or claim against him.
Hope things work out but at least you walked away - which is always a bonus.