Iv just had some issues similar to yours,, car would randomly not start,, just kept cranking but would no fire,,
When it does it next time,, pull one of your plugs and crank it to see if it sparks,, if it doesn't it's defo your crank speed sensor,, if that's not giving a signal to the ecu it will not spark,, same as mine,, but mine wasn't the sensor, it was a dodgy connection on the plug at the ecu end,, recently had it disconnected and bent the pin connecting it back up,,
I did get a crank sensor from europarts it was £19 just to try it,, I ended up putting mine in a vag specialist who Sussed it in less then an hour,,,
An mine never showed any fault codes either, if I unplugged the sensor then it would pick up the fault, but when it was all connected up and cleared all faults, after cranking it would still be clear off faults and still wouldn't start,
Just check to see if you got a spark next time it plays up,, if not, I'd be 100% it's your crank sensor,,