Lost the connection for C/L, left it for a few weeks and it just didn't work when I came to use the car. I don't have the reset codes and the cost of parts is astronomical. I can live with using the key to unlock it, but is there a secret source of these codes or any way to get it working again?

you need a 4 digit code. i'm still not certain if each remote key should have its own code, or if each central locking system should have its own code.

reading the fiat literature suggests each remote key comes with a 4 digit code, and to program any remote into the central locking you need to use any code from a remote that is already programmed in.

when new, your car had 2 remotes already programmed, in theory either of the codes from these remotes will allow you to programme any other remote into the car. if you dont have either code, and a fiat dealer cannot supply one (have you asked?) then you have no option. i havent heard of any other method to retrieve the remote codes from the central locking & alarm system.

you could still install another central locking system but the alarm will not work, then again you could install a central locking and alarm system, its cheap and easy, but your key button wont work it, and the original alarm wont work.

Failing Jugs advice which is usually right there may be some info in here of help.
https://www.fiatforum.com/marea-technical/76180-marea-alarm-fob.html

also see this thread, today i posted the procedure used to reprogram remotes into the central locking & alarm interface.

https://www.fiatforum.com/bravo-technical/90677-remote-locking-problem.html?highlight=code+remote

Thanks for the advice folks: I did ask at thedealer and they can't help if the codes were on a pice of paper attached to the keys: only replace parts at £1000. Car isn't worth it [can't think of one that would be worth that much to fix], so it looks like I am stuck with key operation. I am OK with mechanics but electrics scare me

i've found a pic of a remote key with the paper sticker still on the keyring (code not shown as on reverse). thats what you need. its easy to see how they go missing.

this is a new key which confirms that each remote key has its own paper code sticker when new.