Camshaft or Crankshaft sensor or jumped chain?

[ChilternBeemer]

2006 E61 owned from new! Anyway, it's a great car, always well serviced etc. and without warning decided not to start the other day. No lead up symptoms whatsoever...no unusual noises or performance issues. Mileage 140k.
Engine cranked over but failed to start for the first time in 19 years! The only warning on the iDrive was transmission related, but I know that can come up for all sorts of none related issues..
Called AA recovery to start off with, and the guy was unusually unknowledgeable. Suggested his 'non AA' personal code reader pointed to camshaft synchronization issues. He tried clearing codes but still no start. I didn't think to get a codes read-out I'm afraid.
I have the very good bimmer-tool app and OBD2 v-link but now shows no codes!
I am wondering if this could be a camshaft sensor issue? Or crankshaft sensor issue? I believe the camshaft sensors are easy but crankshaft sensor pretty inaccessible..

Seems unlikely for a timing chain to jump on a warm start with no prior issues?
Any help would be greatly appreciated!