I had a very similar problem with my 2wd. It drove fine, parked up then tried to start it later no joy. Got a mate who is a mechanic to take a look and at first glance everything seemed to be fine, but all became clear after a compression test. No compression in to cylinders. On closer inspection the woodruff key had snapped, so it looked like the belt was turning everything, but in fact it was only spinning the pulley on the outlet cam not the cam itself. Head off to find bent valves and dings in the pistons. The replacement woodruff key was only 11p. I can laugh now, but different story at the time.