Hi there, I have recently bought a ph3 gti and the central locking worked using the key in the door but not using the button. I replaced the battery and it worked straight away, so does your central locking work when you just use the key in the door? If not then you may have issues like a power fault, but if it does work then you need to be looking at the transponder/receiver side I would think. I'm not sure about 306s as I'm quite new to them but the 106 had terrible issues with letting water into the receiver unit via the sunroof seal failing or the aerial seal, maybe the 306 is the same?
Is it infra-red or radio signal your locking?
Is it infra-red or radio signal your locking?