The garage is trying to rip-you-off.
If it's a S/W problem as they say then they know about it and they know that new S/W is (a) required and (b) will fix the problem.
To load new S/W is nothing of a job - just a few minutes at most. £100 cost would be over the top
Create a fuss - get others to create a fuss with you - do not even think about paying the £900 plus 22% VAT
Talk to a no-win no-fee solicitor - make a claim in the small claims court - Citroen have put bad and dangerous S/W in your car
Good Luck..., October 2016