I am the defendant in a case where I cycled my bike into an oncoming car. The claimant is asking me to pay him an estimate of the costs he required to fix the car (carried out by an expert witness-a consultant motor engineer). Why can't I just pay the actual amount the repairs came to as evidenced by receipts etc. How do I go about challenging this?
You are under no legal obligation to pay undetermined costs for repair, even if you accept liability for the accident. You have to pay for the repair, but you will pay once there is a quote for the repair / when the repairs are carried out.
What usually happens (if this case goes to court) is that the claimant, (or more likely, his insurers), would sue you (and your insurers would then defend this action), for the damage you caused. Although they can sue you before they have actually repaired the vehicle, a judge will generally award this kind of damages (called special damages) once the amount has been finalized.
What this means is: you can choose to wait until an invoice for the repairs has been sent, and you then pay the invoice.
There are other considerations as well, such as you only needing to pay the reasonable costs of repair (the repairs have to be at a fair market price, etc.). Because of this it is always best to contact a solicitor.