Unbeknownst to me until today I must have been fiddling around with things on 18 November. Can't recall exactly. Might have been trying El Crappo, dunno...but what I did know this afternoon as that my iPhoto library of 18 November had been rebuilt and lost all photos since 2010.
And because I wasn't aware of this, I happily carried out my normal backup routines. Which backed up the corrupted library.
Time Machine I also kept a TM copy and so went back in time to 11 November and Restored my 20GB iPhoto library (uncorrupted version).
All systems are go. Bloody brilliant bit of software !
Having said that I've just ordered a £20 external drive to do a complete copy... a snapshot of both mine and my wife's libraries.