hi anne.
i think the problem you have with the central locking is probably because the garage you took it to may have disconnected the battery. i cant really see how a timing belt change would have effected anything with the symptoms you describe. as far as i know you cant resync the fob without taking it to fiat but someone may know a way.
its possible theres a bit of a loose connection on the door electrics in the multi plug where the hinges are it may just need disconnecting and reconnecting maybe a quick squirt of WD40 on the contacts. although im pretty sure (90%) the NPG Drivers door 'ecu' is just the window switch assembly and has nothing to do with the lock mechanism but i may be wrong.
in regards to the OBD2 code U1708, google hasnt turned up anything and i have no record of it in my exel document, oh and i searched the whole forum... this is the ONLY thread that has mentioned it ever. are you SURE it was U1708?