Gearbox Failure

I Had my Ford Tourneo custom wagon 2.2 diesel serviced at Brits ford It was the 90K service. the vehicle is only used in town short distances, max 15km per trip. The first trip longer than 50km we took the gearbox failed. the vehicle is out of warranty and service plan. After investigation by the dealer self it was found that there were insufficient oil in the gearbox, no oil leaks or damages to the gearbox where the oil could leak out. The vehicle was sent out to Mayfair gearbox centre to be checked. Same reply from the professionals, no or not enough oil in the gearbox, reason for failure, also no damages and visible oil leaks could be detected. This all happened 2300km after the service. As I have mentioned this was the first time that we have travelled more than 50km in one trip. The dealer states that they do not inspect the oil levels , only when there is a visible leak or damage to the gearbox. It is my suspicion that the "new" Technician that have worked on the car drained the oil and after realising his mistake just kept quite about it. We had to call them the next morning to ask way the "Change engine oil" light is still on. They then sent the Technician to come and reset the indication. My problem is that Brits Ford just stick to their statement that they do not have to check the oil level and that I have to pay the R86000.00 to get the gearbox replaced. No one can explain to me what happened to the gearbox oil. Can you please assist me to resolve this matter.