At that price, replacing them every 9k would probably pay for itself in the fuel you'd save - and the car will run better for it as well

.
Or if you can accept the compromise of no tip projection and a slightly smaller plug gap, fit NGK DCPR7EIX Iridium at around £26 delivered and forget about them for the next 36k.
£62.50 for a set of 4 copper core plugs is ridiculous, though I suppose that includes fitting at FIAT main dealer rates. Easy enough to change yourself in around half an hour or so - and that includes doing the air filter while you've got the airbox off.