Q5 S Tronic Gear Box - Missed gearbox oil change

Hi I am hoping someone may be able to help me, I brought a Q5 3L tdi se in July 2012, from a Mercedes dealers, they sold me the car with 36500 miles on the clock and prepped the car and had it serviced at the local Audi dealers. I have continued to have the car serviced and maintained by the same Audi dealer each year. I had my last service at 66658 in April 2014 but in June I experienced a 'gear box malfunction' when in reverse. I took the car back and they said it was probably due to the fact the car had not had a gear box oil change and this should resolve the issue. A month later the same problem occurred, this time they said it was due to carbon build up in/on a emissions valve, not covered under warranty, so a charge of £1000. One month later again the same fault occurs. This time they say it is the gear box and needs a new one, £9000!!! This would have been covered under the warranty if the gearbox oil change had been carried out when they serviced and prepped the car when I brought it, the service invoice and record sent to Mercedes clearly states that this is due by 40000 but they did not inform me, nor did Audi ever inform me it was required at the 2 services and when I took it in on other occasions for new tyres at 40530 in October 2012, and at all other times. Has anyone else had a similar issues or problem like this, the warning does go out if you park and leave to sit for 10 minutes. Any advice or information will be gratefully received.