Welcome to the forum Richard, you'll find the members on here are a friendly and helpful bunch 🙂
i love the a6 c5, yours looks to be a sport model. 25mpg is Abit low for the 1.9tdi as I'm getting 29-32mpg out of my 330k 2.5tdi v6 model.
the LCD screen failure can be expensive to fix and I'd be taking it to an auto electrician to have the cluster looked at. If you need a new one then it has to be coded to the car regardless if it's new or used.
cheers
Steve