Resyncing fob with car

I just bought a 45 TD and one of the fobs doesnt unlock or lock the car (almost if its dead). I will try a new battery in it but will it need resyncing with the car? If so how do I do this?



Its the old square-ish plektron fob (52 plate) not the newer oval ones (forgot the name of these)



Cheers :)

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