Italian Service? Unburnt fuel? What a load of rubbish.
If the car is only used for short slow speed journeys, then you can certainly build up too much moisture in the engine and exhausts (which is why the back box goes on so many cars), as the condensation that can form, never gets fully boiled off.
Take the car for a nice 60 or 70mph run up and down the nearest motorway or dual carriageway. Don't overheat or thrash the poor thing, but give it a decent run a good fast road speeds and it will 'blow the cobwebs out'.
As for high mileage. I have no idea why we think 100,000 miles is a big deal. In Europe people regularly run their cars to double that.
I did 110,000 in 18 months with one of my company cars, it was on a 3 year / 100,000 mile lease, so the leasing company were not too impressed!
I've only driven a Micra once, I didn't care for it much as it didn't accommodate my 6'1" too well. However, my wifes Peugeot 107 is a super little car and I comfortably drove the last one from Nth Somerset to Halifax and back for a job interview (the company car had a tracker fitted, and I was joining a rival company!).