Rule number one: I have been told it is anathema to admit liability even if you plough into a car at 100mph in a
limit. Submit the details to the insurance companies accurately, honestly and with integrity and let
them battle it out.
Secondly: There are exceptions, but generally if someone hits you up the arse it is
they who are at fault.
On these matters it is usually the norm to direct you to a forum called Pepipoo, but I can't access a link to them tonight and paste it here because God hates motorists.
Google them and take it from there...
(If you Google God.com and get through to him/her/it please ask it to contact me because I have a question or two I'd like answering
)