Re your DTC’s B357A and B57D, seems you’ve got it wrong, there is no B57D so think you mean B357D. In any case both of these DTC’s refer to an auxiliary PTC heater which your car doesn’t have if it’s a standard UK spec.
Your problem could be as simple as a faulty voltage regulator in your new alternator or it may be related to poor connection(s) and/or high resistance within the control and charging circuits.
This is not a comprehensive list but it’s a start:
Simple things first, how clean and secure are your battery terminals?
On the series 2 Antara / Captiva there is a multi-earth point connection on the bulkhead just aft of the vehicle battery. If yours has the same, loosen the main connection, move the wires around to make sure of their grounding and re-tighten.
I believe there was an issue with the alternator plug on these vehicles, you may find the connector is breaking up internally especially if it was roughly refitted at the time of alternator replacement.
Check the cabling between the starter feed solenoid which continues to the alternator output terminal, it’s a particularly cheap and nasty arrangement and prone to poor connection/overheating and high resistance.
