I'm not sure about criminal negligence but to prove other types of negligence you have to show they failed in their "duty of care" (I'm not a solicitor, I just work for one so this is my take on things!)
There's a process you have to follow to make a claim, I guess it would be the same or very similar. The first stage is the Preliminary notice, which is where you basically write to them and state your argument and that you are preparing to make a claim, they have 21 days to respond.
I can PM you a few links to articles on our website that may help? Or most solicitors give a 30 minute consultation free of charge so it may be worth calling a legal expert?