I must admit when I read that the Green Flag had said 'speed sensor' I thought it sounded a bit weird. As soon as you mentioned the car started with a jump-start but the power steering remained off I thought 'battery or charging system'. The power steering is usually one of the first things to die on a Fiat when the battery voltage drops, so it sounds like the dealer is pretty on-the-ball with their diagnosis
Given the
very low mileage, I wouldn't be surprised if the battery is knackered. It must never really have got chance to charge if it's only done 3000 miles in 2 years. However,
given Fiat have just announced the recall for the drain pipe, you might as well try & get a new Alternator out of the warranty, and if the problem persists, go with a new battery, as I dare say two years of water dripping on the alternator won't have done it a great deal of good, even if it's not directly caused it to fail.
Weird that the recall has occurred too - Fiat/Alfa had the same problem with the MiTo and Grande Punto a few years ago, so I'm surprised (though at the same time not surprised) that they've dropped the same clanger again!
If you do need a battery, tbh I doubt that'll be covered by warranty, and being honest with such low mileage & no obvious opportunity for it to charge, it's not really Fiat's fault if the battery has worn out already. If they do offer to replace it FOC then bite their arm off!