The Electricity at Work Regs 1989 say that the competent person carrying out inspection and testing should be able to prove regular training to prove competency (amongst other things, of course).
My 2391 tutor told us that no one has ever defined the word "regular" into an actual period of time but doing the 1 day update courses when new amendments or editions come out is seen as satisfatory.
As for having the qualification for life...well yes you do, but that does not prove your competent. Anyone who sat the 16th edition is still 16th edition for life but does not prove competency to the current edition, thus they must do the updates, etc. Same goes for Inspection and Testing.