A couple of months ago the button to unlock the doors stopped working although the other 2 buttons still functioned fine so i assumed the battery was ok, I switched to my spare which worked fine.......

Until this morning when exactly the same thing has happened to this one!! Any ideas? :S

Not accidentally pressed it loads of times? This will cause the code to get out of sequence with the car and fail to work. That's the only thing I can think of at the moment, unless there's a fault with the receiver in the car which isn't registering the codes and not moving on to the next one.