How much does the 147 drink in average, say 9 l / 100 km?
How much does the 107 drink in average, say 6 l / 100 km?
How much do you drive, 20.000 km / year?
==> you'd save the equivalent of 600 liters of petrol, roughly 700 pounds per year.
If you get:
- a new 107/C1/whatever, you would maybe be able to pay for one extra option with the 700 pounds and spend some other thousands out of your (family's) pocket in difference over a used 147.
- a used 107/C1/whatever, you get an untested car, with an unknown risk of failures, potentially costing far above 700 pounds.
No personal arguments, just facts.
In my opinion, fuel costs are not an argument for replacing a known car with an unknown one, assuming reasonable consumption (i.e. the V6 or above are not a part of this discussion) and reasonable mileages per year (i.e. you don't do 50.000 km in a year or such).