My partner was in a similar situation last year. A car sales guy test driving a car crashed into his then 4 month old Hyundai Coupe. Like your situation he was only offered 10% of value of repairs for depreciation which would have worked out @ €300.00.
He got on to his insurance broker and car dealer who both told him that this is what the insurance company would like to pay, but that he should look for 10% of the value of the car and negotiate from there. He also rang a few other garages who agreed with this.
My advise would be to keep hounding them. My partner eventually got €2000.00 from them (for depreciation and a bit for inconvenience they caused him - long story!).
Another thing you could do is phone some car dealerships and put the following question to them "how much would you give me for my xxx car 2002, no crashes" then ask them how much would you get for it if it had been in a crash amounting to €4k in repairs - use this when negotiating with the insurance company.
The AA have independant assessors.
Good luck.