Shuddering in a DCT generally indicates incorrect clutch friction (outside what the computer expects or can calibrate for). It might be your clutches have degraded, or it might be your transmission fluid is bad (as this has a big influence on the friction in the wet DCT in Mondeo), or it might even be a sensor (as the whole thing is computer controlled).
Two avenues I would start down:
1) Get an OBDII scanner so you can read data from the car. Faulty sensors or abnormal operation should log a diagnostic trouble code (DTC).
2) Check your transmission fluid for quality and quantity. If the fluid is the wrong type or the gearbox is filled to the wrong level, it won't work properly. It sounds like this is what your mechanically-minded mate has already looked into for you.
Rondeo's DCT fluid change write-up:
https://fordforums.com.au/showthread.php?t=11442105
My thread of data-logging and DTCs:
https://fordforums.com.au/showthread.php?t=11452297
Based on your description I would think the prime suspect is the transmission fluid. It has the job of simultaneously cooling, lubricating, and also suspending material worn off the clutches. It requires a very specialised fluid and is very important it's changed at the correct interval.
It wouldn't be unusual for the fluid to look 'dirty' (since it collects all the junk from normal wear) but I can't say what's a 'normal' dirty, except at some point the debris in your fluid would indicate a change is overdue, or your clutches are stuffed. Definitely chase up that prior service and confirm what they used in your gearbox. I'd suspect it hasn't been done or they used the wrong stuff. If you're comfortable with putting in a little money, consider changing the fluid early and see if it makes a difference.
Unfortunately I don't know any mechanics in Sydney, but if you were in Canberra I have dealt with a top-notch shop that I could recommend for you.