Central locking

[Kraims]

Hi, just bought my first A8 D2 2000 3.7 quattro.
As not experienced car buying guy didn't check things out properly as I was too excited to own it and drive it back home...
Central locking doesn't work on doors or boot or fuel cap with key or key fob, only locks driver's door by turning key in door manually, so have to lock each door separate from inside. Using key fob car does flash indicators but not actually locking or unlocking doors. Holding down unluck button all windows does come down so guessing it's not key fob's fault. One question, where do I start? Thanks