Maybe it is just overfilled. If you fill the radiator to the top, then when
the water get hot, like a thermometer, it expands, and comes out from under
the cap. As long as it is below the minimum coolant level mark (and with the
Uno you may need to shine a torch into the tank/radiator to check) it will
be ok. After a few days driving it will settle and so long as it settles
below the Min mark all should be ok.
Some cars/lorries have separate expansion tanks with much more capacity than
the little reservoir/tank that is part of the Uno radiator. Being a separate
tank means that the coolant in the separate tank doesn't get hot and expand
plus the separate tank can have a level sensor working a warning, low
coolant level, light. Think the Talbot Samba had this arrangement. I also,
many years ago, fitted it to my Mini, using a s/h 1100/1300 tank.