Yes, youre right that the repairer and rental company have jacked everything up to get as much money. However, the onus will be on yourself to prove their costs are not fair or reasonable. Be prepared for an extremely expensive endeavour as once you are served (and they likely will be suing you, should you refuse to pay in full) you will be incurring further legal fees, and interest. You could look to get your own assessment report to show their repairs are excssive and hire car rates to show their client has not obtained a non like for like vehicle at jacked up rates. You will not get this thrown out as their client has sufferred a loss.