Hello Mike, welcome to the site.

My Fiesta lacked the remote for central locking and on a rainy day it really did upset me! I didn't look into retro fitting it so am not too much use but I do know that it weigh's in at £200 as a security pack (theatre dimming interior light, perimeter alarm) option so suspect that it is not entirely easy. I would suggest talking to your local service department to see what they advise and perhaps trying to get them to show you the parts required on the parts system. This used to be microcat but I believe they use something else now.

Sorry to not be more helpful.