Electronic handbrake fault after alternator changed

[etaf]

Citroen C4 Grand 62REG, 80,000 Miles
Had a charging issue message on the dashboard
Local garage replaced the alternator, however, on return the handbrake automatic was intermittent would not work.
Took back to garage and they said a lot of fault codes (dont know which ones) are present , they changed the offside sensor, BUT still faulty, they returned the car saying take it to citroen to diagnose, as its showing lots of errors.

Parkign light flashing, handbrake switch does not allow to turn on/off , Parking brake error , ABS error ESG erros cycling through the dashboard info
So booked into citroen for next week
Checked the earths OK and the battery 12.4V off - between 14.5 & 15.5V when running

NOW the car would not allow the handbrake to come off, so could not move out the way of the drive - USED the manual release tool to undo the brakes and moved the car slighlty

BUT still have all these errors

could changing the alternator, or a low batter have caused the issue - the garage have said they told the management system it had a new alternator
just in case its a simple fix - otherwise , its £126 to do the diagnoses check and then £117 an hour + parts to fix

thanks in advance