Fitting the bypass relief valve was carried out as a factory recall and all owners should have been contacted (at the time) by the DVLC.
VW has a list of the cars on which this modification has been carried out and will perform the modification for free if it hasn't been done. I do not believe there is a time limit as this is an essential safety modification. Why would anyone want to diy when VW will do it for free ? Contact your dealer to inquire if your car has been modified. kind regards, yt.