Top job, there will be a lot of main dealers who would struggle with that.
At the risk of damning with faint praise, I wonder if 'I do my own servicing' has something to do with it.
Some types of this gearbox require an occasional oil change, which costs a lot because the lubricant has to be pumped in.
Inevitably, home mechanics don't do that because they lack the knowledge or special tools required.
On t'other hand, some versions of the gearbox were poorly designed.
I believe Ford in the USA lost a class legal action over it.
The USA class action is over the DCT250 iirc which is dry clutches and shockingly bad!. Fiestas etc had them (250nm max torque).
I bought the car at 55,000 miles with "full service history". Gearbox is due oil and filter every 37500 miles or 3 years.
I did the gearbox service when I bought it with Genuine Ford Oil and filter (£160 kit), did it by the book including final level setting (if you ever remove the level plug on a 6dct450 you HAVE to refill approx 0.5l, very quirky setup!.).
Many dealers have arsed this up or guessed on fill levels so I did it myself for peace of mind.
Picture below is the oil that came out at that service.
Done 40 miles mixed town and 60mph bypass driving this morning, hasn't missed a beat so back to my wife to use tomorrow hopefully!.