Key fobs/central locking on my GTi

Alright everyone, been ages since I've been on here but I've come crawling back with a problem :-D Recently my key has stopped unlocking/locking the car, I've tried changing the batteries in the fob and il get a little red light to indicate the batteries work but if I press the buttons it does nothing, my spare key gets a red light when I press the buttons but for some reason it won't unlock the car. I've been having to insert the key to unlock but if I wait too long before starting the engine the alarm goes off and the same happens if a passenger gets in. Anyone had this problem or have any ideas? Sorry for the long post but it's a real pain in the arse having the missus glare at me every time she gets in with the alarm ringing in our ears lol