Sorry to hear of your experience Harrow, hopefully you both weren't injured too badly, but I'm sure you must have been badly shaken by the accident, I know I would have been. The last you need after £$%^ hits the fan is a non sympathetic insurance company.
With an accident like yours I assume the police and other services maybe were involved and they can all verify the accident cause, so blame isn't likely to be disputed.
It's really important to 'stick to your guns' when negotiating with the insurers and as you have a hire car I assume you don't have a desperate rush to replace you lost pride and joy. Hopefully you have written to them pointing out all the positives of you lost car.
I think some of these Free Legal Cover insurers may be no more than No win No fee companies, so make sure you only sign up for what you understand and that they wont come back to you, if they don't win.
Good luck and be pedantic in claiming all your losses, inconveniences etc. , I'm sure mentioning the insurance ombudsman if things are going fairly may help.