If you are happy to keep the car, i would take it back to them and say, you sold me this car under the impression it was an ex demo as it is an ex hire car, tell them to do all of the usual engine checks and change any parts that tend to wear out quickly along with throwing in a full warranty if not already done so.

Also, tell them that if they are not willing to do this then they can have the car back for a full refund.

If they say no, go straight to Alfa uk and deal with them.

If this deosn't work then, i'm out of ideas.