It is a great idea by the National Theatre; and one that will generate charity donations for the theatre at this difficult time. The Royal Opera House and Andrew Lloyd Webber (musicals) are also putting free performances on youtube.
My wife and I were disappointed by 'One man, two govners' after all the hype. We had seen the original play on which it is based years ago and found the original side-splitting. We saw the modernised version at a local theatre and were disappointed and we were again last night. Corden and the cast were good, as you would expect from the NT, but why did it need to be modernised (to the 1960's) and as it was, why not use some songs of the 1960's at the scene changes rather than the increasingly tedious 'modern' versions'?
We are clearly in the minority on this play so don't let us put anyone off watching it. If you haven't seen it before, maybe this will reduce expectations and, expecting less, you will love it!