I did nod drive my car for couple of weeks so it was connected to smart charger to keep battery in good condition. When I decide to do some drive and enjoy, I disconnect charger and went to start the car and then surprise. Starter hardly manage to turn...
Than I put this small voltmeter in socket and see 10.7 Volts on my surprise. Eventually car started with some warnings, I did run it on idle for a while and 14.9 Volts was on voltmeter. Than I went for short trip and went back stop the car and start again and starter was struggling again, with 10.5 Volts on voltmeter.

Well, it's busted, one cell was fried - I told to mayself, and battery was 3 years old.

I write this because I saw similar complaints when everything looks fine , alternator and starter good, battery good and suddenly problem with starting. Just put voltmeter while charging and than disconnect charger and read the numbers. Mine dropped from 13.8 with charger on to 10.9 with charger off within 5 seconds. Than you know you need a new battery.

I had the same with my last battery. It dropped a cell and would show as charged on the Ctek but fail to start to engine. New battery required. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed