How can I fix my 1994 Audi A4 central locking?

I will start by saying that I do not have a remote but use the actual key for locking my car.
My central locking has developed a fault where if I lock the car from the drivers door the alarm will set but the buttons do not go down so effectively the car stays unlocked. The same thing happens if I lock the car from the boot. However if I lock it from the passenger door then that button drops but the rest stay up. Also if I press the internal central locking button none of the doors lock. I have also just noticed that whenever I try to lock the car I can hear a buzzing or humming noise from the back of the car for about 5-10 seconds. Oh, and the petrol filler flap never locks anymore. Any assistance would be great.
Cheers
Rik