Key Fob Problem
CITROËN DS3

I have a 2016 (not on list so used just to identify DS) DS DS3 1.2 Puretech Elegance which I bought last August. For a couple of months the keyfob remote lock/unlock worked fine then stopped. I assumed dead battery so took to Citroen for new one. They would not replace as said small plastic “tang” which stopped kew from opening was damaged so needed new key at cost of £300. I then replaced battery myself which worked perfectly for 3/4 days then stopped again. Replaced battery 2 more times, same problem. Since then I have used key manually which unlocks and starts car normally, When locking, it locks car, sets alarm and immobiliser without problem. Could this be a transponder problem, if so no alternative but new key. Any ideas would be most helpful. Thanks.