Update:-
Torque wrench to 40Nm and so far so good. Orbea support totally dismissive/unhelpful and all they could supply was a link to E-13 website (already found and where no surprise all it says is 8mm Hex and Tq to 40Nm) User Manuals – ethirteen support
Bosch has no detailing either, however...
Same problem here yesterday (bike 6 months old). Having left the hills, heard crank hit road, fortunately bolt was nearby. Intend to raise with vendor & Orbea. Their website doesn't list the bolt as a specific spare however I have found some aftermarket items- not sure if link posting allowed...