My wife bought a 12 plate fiat 500 a year ago. The engine warning light came on after a couple of weeks so we took it back to the dealers. The heater overflow tank was cracked and levels were low. The assumption was this had caused the issue.

The warning light still came on and we got limp mode quit often. The garage took it in and put it down to the throttle cable sensor. Meanwhile the heater decided to only blow cold.

Everything was fixed and we had hot and cold air. All covered by the seller. Two months later the heater is only blowing hot. Cold is not an option. I know several people who have had this issue and I believe the fix to be several hundred pounds and lots of taking stuff apart on the dash.

Does anyone have proper information?

Do you have climate control?
Or the normal hot and cold manual system

The heater flaps can break internally. Its a common problem on climate control heaters.

But the manual control heaters can also refuse to blow fully cold so it seems they can also have heater flap problems.

Another possibility is low coolant which risks (or is caused by) a blown cylinder head gasket. With your history that's quite possible.