Help - Warranty Problem

[cheekybee]

I recently bought an A6 1.9 TDI from an independant dealer. 1500 miles later and the gear box has gone.
Something broke in the gear box (not wear and tear) cracked the casing oil has leaked and the box has heated up and seized.
The car has been regularly serviced by audi and has done 108000 miles. The warranty company had an independant engineer assess the damage - who states that "The driver/operator would have been unaware of the of the initial failure and resultant (oil) leakage"
though they follow this with "The operator/driver would have become aware of the condition by excessive noise and difficult gear selection"
No noise or problems were apparent, just instant failure. I then called roadside assistance to have the car examined. Can anyone offer suggestions as to how to best present the case to get them to pay the bill? It appears that only their comment about me not noticing the damage is causing them not to pay - but why would you have your car examined if there was nothing apparently wrong with it?