Did you buy the car new, or used? Either way, it should still be under warranty.

They need to fix the fault as an airbag light is an immediate MOT test failure - rendering the car not roadworthy.

If the car has been at the dealer's several times over the past year, they have had an adequate chance to rectify the problem and have evidently been unable to do so.

Therefore I think you could reject the car as being fundamentally faulty. To do this, follow Honest John's advice to the letter. It's all here: https://www.honestjohn.co.uk/faq/consumer-rights/

Basically, you write to the senior manager of the local Ford dealer, telling them you reject the car because of the fundamental safety issue, and detailing (with dates) all the times you have taken the car to them to have it fixed. Send the letter recorded delivery, keep a copy, staple the proof of postage to your copy so it is a matter of record.

If you want another similar Fiesta, insist that the dealer finds you an equivalent car with equivalent mileage. If they won't, then they need to cancel the PCP at no cost to you because the car is fundamentally faulty.