Procedure to re-sync remote key fob

What is the method for re-syncing a remote key fob for a Fabia Mk1. The key fob has all of a sudden lost the function to remotely unlock or lock the car. The key fob battery has been changed, I actually tried two new batteries and when the button is pressed the red LED illuminates, it also did so on the original battery. The key will start the car, also the spare key works as it should. I used to have a print out of the procedure to re-sync a key fob, I had to do this previously, but that was a few years ago and I've mislaid the printout and can't remember how to get the remote working.