Hello, I hope I can offer you some assistance.
As the car is charging OK and the battery is new then there must be a current drain when the car is locked and left.
You need to fit an ammeter between the battery positive lead and the positive terminal, make sure the meter can cope with the current. Latch the bonnet lock with a screwdriver then lock the car, after approximately 40 minutes check the ammeter reading it should be less than 0.05 amps, any higher will indicate a fault. If there is a fault then you need to pull individual fuses one at a time to isolate the circuit causing the drain.
Let me know if I can assist any further.
Best wishes for the season,