As MEP pointed out, yes, you would be entitled to leave it for two years as long as you had it serviced within the acceptable limits of 18k, but again, why any private owner would leave their vehicle for two years without servicing it, is beyond me. Ahh, yes, I've got it, they are merely following the manufacturer recommendations!
Extended service interval times were introduced to appease fleet buyers who got rid of their cars every couple of years. With our new TA, I'm having the oil changed at 3k, regardless of the fact that the first service isn't due until November this year. People out there will think that is stupid, well, they are entitled to their opinion, but for the sake of £12 for a genuine Fiat oil filter (which I've already bought) and 20 quid for 4 litres of Petronas 5w40, it's hardly going to break the bank. Plus I think it is important to have an oil change on a new vehicle at 3k once the engine has bedded in.
Going back to the service book aspect, as others have rightly pointed out, a stamp in the service book doesn't exactly mean much, you must have the receipts to go with it. Indeed, when I did the PX on our old POP with the Fiat dealer, he was insistant that I provided receipts of evidence of work carried out on the vehicle, which I was able to provide. He wasn't bothered in the slightest either, that the car had been serviced by an independent Fiat specialist, the invoice clearly detailed what had been done to comply with the warranty terms. If you are having your vehicle serviced by good responsible garages, they will provide clear invoices and there really shouldn't be any trouble deciphering what has been done. I've still got copies of every invoice for every car I've ever owned and I can see exactly what has been done.
If you don't have a service book and the dealer you bought the car from won't give you one, send an email to Fiat CS and ask them to send you one.