This is what I would do based on what you have said:
Go and take pictures of the scene
Print of google maps of the road/junction etc
Take pictures of you g/f's damaged vehicle
Get the TP to confirm their vehicle was involved in an accident
Request pictures of TP vehicle
Get your g/f to fill in an accident report form and request this from TP
Get a report from the repairer confirming the damage is consistant with the circumstances
Once your insurer has all of the above get them to write to TPI and explain only way the damage could have been done to your g/f's vehicle would be in the way described.
Do not concede liability or accept 50/50 no matter how much your insurer tries to convince you.
It is standard practice to deny liability in any claim regardless of circumstances. A bit of arguing and numreous letters will result in your insurer threatening legal action and then the TPI will probably concede liability. Your insurer may even have to instruct a solicitor, which usually gets the TPI conceding as legal costs usually outweigh any sort of claim they have to pay out.
Hope I haven't missed anything out. The best thing is to keep on top of your insurer and constantly chase it.
lol if only it was that simple.
Unfortunately the damage wont proove who started the manovre first.
TP can claim that they were performing a safe manovre, and the OP in question came racing along speeding and crashed into them and didnt signal.
In which case the damage on the OP's car neither confirms nor denys this so the TP refuses to accept liability and it goes 50/50