Where was the MoT carried out? If it was at the Kia dealer that sold you the car then I'd suggest taking this up with them rather than Kia UK. The service manager at the dealer may be more sympathetic to your case and more willing to retain your business with the offer of a free or discounted repair.

Ultimately, if Kia refuses to budge then there isn't a great deal you can do to force them to change their mind. This is not a common fault on the Picanto (it's the first case we've heard about). I'd recommend taking the vehicle to an independent Kia specialist.

Has the car experienced a flat battery recently? If so, it may have depleted the back up battery for the airbag and it will need recharging and a master reset to correct the problem.